The only roller coasters my girls do are Thunder Mountain and the Goofy one in Fantasyland. Thunder Mountain has a transfer seat that folds down for an easy transfer. Goofy's last row has a wider opening.
He should request to load upstairs on Test Track and request the booster seat making the transfer easier. They also have a transfer bar if he needs something to pull himself over with. (We have never used the transfer bar but it has been offered to us before).
Also on Kali River Rapids he can request the transfer steps that will allow him to slide over from his chair and bump down the steps into the ride.
Both of my daughters are paras and we spend 6 months of the year 8 miles from Disney and go a lot.
Funny - we have never used the booster set, but have used the transfer bar.
My daughter has cerebral palsy and we have to lift her into ride cars. If I am alone, she can do a stand and pivot transfer, but my DH likes to do the brute lift.
She has not been on the mountains because her muscle tone would make it too difficult. Of the thrill rides, she has been on Tower of Terror, Test Track, Star Tours, Dinosaur and Kali River Rapids.
One I would STRONGLY recommend NOT going on is Primeval Whirl. It is like a roller coaster combined with the teacups and look deceptively tame from the ground. Because she can't brace herself with her legs, she slid down and was nearly off the seat by the time the ride ended.
Here at some pictures for you:
This is the 'seat belt check' area that other posters mentioned for boarding at Test Track. The ride has been re themed, but the boarding area is still the same.
This is the accessible boarding spot for Kali River Rapids. Follow the instructions in the Guidebook for Guests with Disabilities or the CM at the entrance to the queue for where to go.
As an empty boat goes by, the CM opens the gate (the bar going across the water in the middle of the picture). That traps a boat so that it will not move until all the guests have boarded.
For exiting, the CMs do the same thing.
It is not an easy transfer. A wheelchair can be parked next to the boat, but you have to go down into the boat and then over to a seat. The seats are made of plastic and the seats and floors can be wet and slippery because of water coming into the boat. Once you have gotten into the boat, the CM will move your wheelchair out of the way. You will return to the same dock when your ride is over.
Tower of Terror ride elevator and waiting area. The wheelchair can be pulled up to the elevator door, then you transfer to a ride seat. During the ride, the wheelchair will be brought to the exit.
Tower of Terror ride seats - they have individual seat belts and grab bars.
Big Thunder Mountain RailRoad. Since I was not with my daughter, I boarded in the regular boarding area. You can see guest's wheelchairs parked in the area where they boarded in the background of the picture.
Big Thunder Mountain special ADA car (thanks to Bill Sears for photo).
expedition Everest trial car (thanks to Bill Sears for the photo).