Retts syndrome is very dibilitating. I've seen a girl with it she was 12 - they have problems with using their muscles . She couldnt walk and had leg braces on to keep her legs straight while her mom tried to move her around places. She couldnt move her hands
or her body at all.
Genetic doesn't have to mean direct inheritable - for example, autism might be a complex set of genes. In this case, both you and your partner would have some of these, and your son recieved the full combination.
This doesn't mean that you and your husband are both "a little bit autistic", the same way that flour and eggs aren't individually "a little bit cakey".
I probably shouldn't be saying this, as it's based on very little evidence - but another option is that your husband might want to look into the aspergers thing, if he's interested. From some of your earlier posts, I remember you mentioning autistic traits in his family, and the fact that he's a D&D player (it's a bit of a stereotype, but RPG's are a very common aspie interest).
My husband plays several RPG's (Not just D&D), I didnt even know what the letters stood for but I asked him LOL. He completely looses track of time when he plays too. He is very much into video games of all sorts. He's somewhat of a child .He is also a computer "geek" . He builds them and works with them - He works in computer networking. I forget his exact title 
I doubt he'll look into aspergers though - or how should he look into it?
I have thoughts about one of his uncles having AS. I dont know for sure. I hardly ever see him at social functions to tell but perhaps that is an indicator. He's in his 40s I think and was never married. When I do see him you have to talk to him first - he never approaches you for communication. He is the only person I can see as a possible though.