Hi Tracy,
I think Disney does a good job of working with guests to have a special time on vacation. This is especially true for guests who may need a little extra pixie dust.
There is a cast member at the beginning of every attraction start. If he needs to get off, you can approach this cast member for assistance and they will direct you to an exit. However, once the rides starts and he is on the ride, he will need to continue. My own daughter does not like some of the scary rides. I have had to tell her close your eyes and I will talk you through it. However, this is good in theory but may not always work.
Two rides I would avoid are
Stitch's Great Escape in Magic Kingdom and
Dinosaur in Animal Kingdom. The rides are scary and there is no way to get off of the ride.
I would have your son look at the website at the attractions and talk about them. You could also order the planning DVD and watch it as well. There may even be YouTube videos on the rides.
Wishing you a magical vacation!
Beth