Like others said, there is a lot he might not be able to do, but I think he could still potentially have a good time. Is he a Harry Potter fan? If he is, just walking around both lands and window shopping is great. He can get a wand and go around performing spells. Just trying to get to all the spell locations will take a couple of hours. And queue line for HP and the Forbidden Journey is great, it's like a walk through attraction all on it's own. It sounds weird, but you could totally enjoy your time just walking through the queue line and seeing everything. Ask a CM at the front of the attraction, this doesn't always happen but sometimes they will let people walk through the Express Line just to see the queue, and from their they can very easily exit without getting on the attraction (although as a note there are stairs). Also, I know Despicable Me has a non moving seat option. Like the OP said, you're basically in a large room and everyone sits on a moving bench. You can ask for the non moving seat (which is also the option for children who don't meet the height requirement) and basically enjoy the attraction like it's a movie. It's a big enough room that hopefully you wouldn't have an issue with claustrophobia, but if you all walk in and find it too constrictive they will let you walk right out the exit. I think some other attractions have a non moving seat option, at least Shrek 4-D did before it closed. And how does he do with Splash Mountain and Kali River Rapids? There are some similar attractions at Universal (Jurassic Park, Dudley Do-Right, and Popeye) if you want to risk getting wet.
Might I suggest checking out the disabilities guide on the Universal website? It does a pretty good job describing the health restrictions and motion of each attraction.