Hi,
Depending on where his interests are and whether his medical conditions may impact his abilities to handle smaller stuff there are a number of snap together kits that actually build into pretty nice looking models. Recently someone here had a thread going on an Academy Hyundai Azera kit that really didn't require any painting at all (unless you want to) and actually looks really nice (I also have a copy of the kit waiting to be built).
Similarly, Revell has a couple Police cars and NASCAR cars that are easy to build and looks fairly decent (which I believe may come with peel and stick stickers instead of decals), as well as some other snapkits like a Ferrari, 2010 Mustang GT convertable, 63 Corvette Stingray, a Humvee, a New Beetle and some sort of Chevy 4x4 pickup, etc. (http://www.revell.com/model-kits/snaptite/ )
Kits like these can be a fun way to spend time because they typically aren't too expensive and you can put as much time as you wish into them (either just doing a simple build or spending more time by adding detail and painting etc) while still having something to show at the end.
Hope that helps.
Pat