Has your son been tested for hypothyroidism? Hypothyroidism often causes steady weight gain and it often goes hand in hand with other conditions.
Have you ever looked into yoga for your son? Yoga is one of the best types of exercise for people with all types of conditions, because of its low impact on the body. With yoga, you can start out at a very low intensity and it's not necessary to keep up with the "instructor" - you do what you can, as well as you can do it... when I first started doing it, I thought I'd never be able to do strike some of the poses, but it wasn't very long and I could it without a lot of pain. You might want to do some research on it. It's something that can be done at home to videos - you can watch them free on YouTube, if you like or buy inexpensive dvd's. It's also something that they whole family can get involved in.