He rode Big Thunder Mountain 11 times on our last trip. Expedition Everest 7 times. Primeval Whirl 4 times. Soarin' 3 times. Test Track 3 times. Splash Mountain 1 time. Tower of Terror 1 time.
I was never afraid he was going to fall out because I was sitting next to him both times. An adult and two kids (both 7) sat comfortably on Big Thunder Mountain, Expedition Everest, Primeval Whirl and Test Track. On Soarin' we were sitting individually but we were strapped in extremely well.
I won't let my son ride Space Mountain because it is a single sitter ride. Rockin Rollercoaster probably won't happen either.... my son is too scared of the loops.
Have fun!
My DD has CP, but she is not a walker and goes into extension patterns when she is excited. She has mixed CP - spasticity in her arms and legs, tight hips, but hypotonic in her trunk. She also has some athetoid movements. (quite a combination).
She has been on Tower of Terror once, Test Track many times, Soarin many times and Barnstormer.
I think that Soarin is one of the best rides. You are in a ride car that is kind of like a lawn chair - a little reclined. There is an additional strap you can put between the legs. With that and the seat belt on, you are not going anywhere. Besides that, the ride is very gentle movements (you are on a hang glider, after all).
If he likes Star Wars and wants to go on that, it is pretty jerky, but DD likes it and doesn't have much problem. You can request an extra 4 point harness, which gives a belt that goes over both shoulders. That helps a lot with trunk control.
Another ride DD enjoys is Kali River Rapids. At the regular boarding area, the boats and the dock are sort of moving together. They do also have a wheelchair boarding area where the boat can be 'trapped' so it stays still for boarding. You share a seatbelt between 2 people, but it was not too hard for my DH to control her - the seating is 2 seats together.
My DD has also been on Mission Space, the GREEN (tamer) version. The less intense version is still kind of wild, but less than the full intensity version and it doesn't spin.
DD has not been on Big Thunder Mountain, Expedition Everest or Rock N Roller Coaster because we were afraid we would not be able to control her extension. I have been on them (except for Rock N Roller Coaster) and feel they would be fine if it wasn't for the extension my DD has. Maybe sometime we would do them (we have to feel up to holding her down).
We would never consider Space Mountain for the same reason as MightyMom - the seats are one in front of the other. I rode on Space Mountain with my other DD when she was 5 or 6 and it was one of the worst experiences of my life. She was afraid and I couldn't do anything about it since I was sitting behind her.
The only ride we have ever had a problem on was Primeval Whirl. I can't ride those 'spin-y' type rides, so she rode with her older sister and her dad. Both girls were well above the height requirement, but both are thin. As they came around the track, I could see DD sinking lower and lower in her seat. By the time the ride came to the end, she had slid almost out of the seat and all that was holding her in was the lap bar which was up at her underarms. Watching her, I was really afraid she would fall out. My other DD said she thought the biggest problem was that her sister could not use her legs to help hold herself up. Neither of my DDs will go on that ride again because it's too dangerous for youngest DD and oldest DD says it's way too rough.
I'd suggest maybe having someone else in your party ride Primeval Whirl first. It is a lot rougher than it looks.
Also, if you want to have a sort of 'test' roller coaster, I'd suggest Goofy's Barnstormer. It is a good test in terms of the motion comparing with Big Thunder, but much shorter.
Allearsnet.com has a page with ride car pictures. Most of them don't show the whole ride car, but they do show the seatbelt or restraint.