For those who helped me out, this is just an update on my son's vision problem. He was tested by some behavioural optometrists, and it turns out his biggest problem is that his brain is ignoring info from his right eye (he is left eye dominant), so effectively it is like he is blind in one eye while both eyes are open, though when his left eye is closed, he can see out of the right one. It's called a convergence difficulty, or perhaps he has a convergence problem because of his brain ignoring the information from one eye. Anyway, the problem started way back, probably at his birth as there was some trauma then, and his brain decided it was easier to ignore some info, rather than put it all together and try to make sense out of it.
The simplest test for convergence problems is whether both eyes follow an object, like a finger, when the object or finger is brought towards the subject's nose. The eyes should obviously squint towards each other(convergence). If there is a convergence problem, the dominant eye looks at the finger, while the other eye tracks off to the side.
Anyway, they are going to put him on a programme of exercises to retrain his brain, hopefully. I have found 2 other people who have this problem and who has been helped by the exercises, so here's hoping.
Maybe this will help someone else, too.