Hum, that's a tough one. I'm sure he has been working with a physical therapist at some point in the past. Maybe the family can contact the therapist and ask them for advise. The PT would have assessed his trunk control and know what limitations he has. How is his cognition? If he can answer questions he can be involved with some decision making about if he wants to go in some of the ride lines then not ride. Gringott's is neat to walk through and watch the pre-shows even if you don't ride. They let you out the door right before the steps if not riding. You didn't mention how old he is or if he watches movies. If he is or has worked with a Speech Language Pathologist they could explain more to the family about his cognitive level and how much he comprehends. He might be fine just going around the gift shops and the things around the areas and not worrying about the pre-show/ride lines for things he won't be riding.
Minions, Shrek, and Terminator all have wheelchair areas that don't move. Nothing to see at RRR. There is a cute preshow at Jimmy Fallon were they rap the safety warnings - not sure if they would let him watch that, plus the waiting area has dancing Hashtag the Panda and other singers. The Mummy has a neat line that might be fun for him to go through - even the wheelchair line then not ride. MIB has some pretty intense spinning so that is probably out. Again the pre-show might be enjoyable but is skipped for people using EPs. Not much to see in Simpsons ride but the area down the street has some display type characters that you could take pics with while others ride. I've never been in the Woddywoodpecker area except for ET and that line is cute but he would not be able to ride unless there is some modified car I'm not aware of. Obviously things like Horror Make-up show, Sinbad, Posinden's Fury would be fine since they are shows.
I'm not sure about HE (Harry Potter train). I know families are not allowed to bring strollers into the carts but they store them at the end. There would be no space in the regular train car for a wheelchair that someone can't transfer but I'm not sure if there's a modified car. The line is still neat and IMO is half of the fun of riding. Not much at IoA I can think to ride since Spiderman is like Transformers and no bench type seating. The line through FJ is neat to walk through then not ride.
There really is so much to look at everywhere. Not sure if he would be able to use the wands as far as vision, following instruction, and arm coordination. If unable he may enjoy watching other kids use their wands for the tricks. The workers at the front of each entrance should be helpful as far as allowing him and a helper to walk through the line and look around while others ride. If he uses the EP it will skip most of the interesting part of the lines for the rides.