The most likely cause for this change in your son apart from his adolescent hormones is something happening at school eg. bullying or personality clash with the teacher. It's worth investigating that angle and also checking if anything has happened that's different eg. has he lost a good friend or favourite teacher recently? or you have moved house recently?
Sometimes, a seemingly minor change in routine can have drastic effects on an autistic child. He might also feel that because he can't have sugar, he is marked out as different from the other kids and they could be teasing him.
As long as he isn't diabetic, it might be better to at least let him have the occasional chocolate or soft drink so he won't feel so out of it. Unless putting him on the special diet made his symptoms a lot better, it's probably not worth the trouble.
He could be rebelling against his every move being recorded. I think even kids without autism would but then I realise you need to know what is triggering off his outbursts.
Sugar free and dairy free diets would mean a lot of foods most teenagers take for granted wouldn't be "allowed" and this could be causing him frustration. Has your son ever said he wants to have these foods or is he okay about the restricted diet?
Then again, if he is at a special school, other children might be on similar diets and so he wouldn't be so out of it.
Has anyone at his school said anything ie. noticed a worsening in his behaviour at school? Still, some kids will bottle up all the difficulties they have outside of the home and then "dump" them on the people they most trust ie. their poor families at home.
I've had a few meltdowns in my time (mostly ironically when I was an adult).
There are some things that always make them worse for me:
(1) Trying to reason with me and/or say I'm being unreasonable.
Maybe it's true but it's not the time and place to say it - it's got the same effect as dumping a can of gasoline on a fire.
(2) Saying I'm using emotional blackmail (because I'm not and wouldn't).
(3) When I'm starting to show signs of agitation, trying to force me to do something I say I can't do/am too scared to do/acting confrontational.
But things that make it better are:
(1) Holding me tight.
(2) Asking if I would like a drink.
(3) Giving time to calm down and then asking what the problem was.
I'm always very remorseful about having the outburst.
Of course, I realise your son might not respond in the same way but maybe something in this can help.
Kids can get what is called "constipation diarrhoea": a "plug" gets impacted and then runnier poops slip out around it. The kids often have "accidents" and it isn't until they get bad cramps and get seen by a doctor that the real problem is found.
They might also react by hiding as they are embarrassed by the accidents and worried they will get into trouble for pooping their pants.
I'd say soy products would tend to constipate if used in large quantities.
Fresh fruit and veges and grain bread would be better but I suppose not all kids will eat these things.
Soy gives my family (3 aspies) the runs, well, not it's not so bad to me but my mother and my husband can't really eat much of it at all.
Too bad, as we had just found the BEST tasting veggie-burgers...
He's very big to still be soiling himself but if he has got some intestinal problem, he probably can't help it. I don't think I would be so patient unless I were sure there were some physical cause (but as I am AS myself, maybe I don't have the same level of understanding and patience as others).
Well I don't know about anyone else, but I certainly wouldn't eat something that I knew (or had a very good idea) would give me the runs. I have a bit of a phobia about that kind of thing. It's a nuisance as I can't eat my favourite yoghurts any more as I know I will suffer for it.
Mollie.
You are not alone, for a fact. I read posts of many, many other parents in different places about temper tantrums, meltdowns, and even agressivness. Oasis for one place. Just praying won't end it, so keep up with the helping him. And, I'm not saying to change who he is completly, just to help him out and not to give up. There are plenty of other moms that feel like you do.
I used to have outburts quite frequently, I bit my teacher once... Although violence was very rare in my outbursts, I often would damage stuff by throwing it around and stuff. Hmm... My outbursts were always triggered by somthing, Although they seem so random at times, They'res most likely a simple cause to it. Most of my outbursts was simply caused by stress, Or anger. Usually they only lasted 2 hours. Try to eliminate what might be causing his meltdowns. Hope this helps
I can definatly agree that there is a trigger to many metldowns and outbursts. The trouble is finding what it is. My parents claim I had horrible outbursts, but now I wander these days. They have egsagerated it a few times. They didn't seem to understand my distress, or my frustrations. At the time, and from my teen view point, it seemed like they only saw an angry brat that needed to be controled or changed. Ow. My father prooved the myth to me with his epesodes of shouting degrading things in my face. My mother didn't know quite what to do.
It made me feel worse thinking he hated me. After a while, he would 'correct' me and punish me when it was as minor as grunting or for not at all even. My mother however, has treated me so much better, and finally understood that I had been upset in my teens because I suffered from school bullying and isolation. AS makeing some of the isolation very easy. I was more scensitive then. The slightest, and I mean the very slightest offence would tick me off really bad.
My mother was with me trying to figure out the whole problem that we both didn't know about. My father whined a little bit about the prices of prozac and councelors, even though my mom was paying for them. They gave me the prozac for depression, which I didn't have. The jerk I spoke to upped the dossage just because I didn't reply like a dumb blond, happy dandy NT. That kind of perscriber must be watched out for! My past quak got sued out of business for some other problem, Thank God!
When she realised I was only getting high from it, we both wanted me get off it, and didn't see that quak again. We left the counselors finally because they did nothing for me. What I needed was someone to tell me that I was smart, pretty, and other possitives because I had limited to nothing of that. I couldn't find certain ways of explaining my bottled up hurt because I didn't know I had it.
Yep there were reasons. They thought there weren't any reasons for my anger because they had no clue. Getting high is a bad thing. I hope 'getting high on perscription" doesn't happen to the child in question! Oh goodness, I hope that doesn't happen again.
How possitive has his life been and has he heard something loving enough times, or is he just supposed to be a certain way? What is that trigger that sets off unpleasant feelings? If he beats himself, chances are that part of himself hates himself for emotionaly normal reasons.
I rarely got really aggressive as a child but suppressed a lot of anger and it then came out as an adult. It might have been better had I acted out a little more as a child and got it out of my system.
When I did get aggressive, trying to "reason with me" was useless - I responded best to being allowed to go somewhere quiet and have a good cry.
I have the same thing with my 14 year old daughter she is getting aggressive toward all members of the family including our dog. I came home from shopping recently and she had my wife bailed up in the kitchen with a knife. I was just lucky that i happened to walk in at the right time to disarm her as i fear she may have stabbed wife.
What concerns me more is that we are adults and can fend for ourselves but what happens if she takes the same course of action with her younger sister who is only 8.
It is a very scary thought.
Have her tested for Bipolar ASAP - about 40% of people on the autistic spectrum have some degree of Bipolar and that sort of stuff is very typical.
I don't believe we're talking about poo. But here's my contribution to the big cacka debate.
I have had diarrhea for my entire life. Can't remember a full week when it wasn't a problem, and I've tried every diet, vitamin and herbal remedy under the Sun. By the time I was 15 I was used to the pain, the cramps and the inconvenience - and I just managed it and ignored it. But I have never *ever* shat the bed. I think you should take him to the doctor asap.
It's a statistical fact that serious medical problems are often overlooked in individuals who are neuro-atypical because the autism label is invoked to explain behaviours and warning signs of a actual medical condition.
Alectrum, I can sympathise as this would be such a miserable condition to have. Have you ever been checked out for IBS or Crohn's disease or even gluten intolerance as constant diarrhoea is often caused by such conditions.
Maybe Mollie meant her son was starching his sheets and I got the wrong idea that she meant he was pooping his pants instead.
It's probably IBS and Gluten intolerance. My mother has both. The things is - I'm used to it. It's unremarkable. Because I've always had it, then I have a high pain threshhold in that regard. I ignore it and block it out unless it is extremely severe, and even then I have coping mechanisms.
I'm not sure what Mollie meant, but I have two younger brothers and I did their washing when I was younger. If they're male - they stain the sheets. If I was Mollie I think there was something wrong with him if he didn't starch the sheets at his age! 
Alectrum, I can sympathise as this would be such a miserable condition to have. Have you ever been checked out for IBS or Crohn's disease or even gluten intolerance as constant diarrhoea is often caused by such conditions.
Maybe Mollie meant her son was starching his sheets and I got the wrong idea that she meant he was pooping his pants instead.
It's probably IBS and Gluten intolerance. My mother has both. The things is - I'm used to it. It's unremarkable. Because I've always had it, then I have a high pain threshhold in that regard. I ignore it and block it out unless it is extremely severe, and even then I have coping mechanisms.
I'm not sure what Mollie meant, but I have two younger brothers and I did their washing when I was younger. If they're male - they stain the sheets. If I was Mollie I think there was something wrong with him if he didn't starch the sheets at his age! 
I just hope he never never ever reads these pages. rotfl
There's also the fact that he may be thinking about things that are making him very angry or despairing.