Since I know you mean Forbidden Journey, I want you to know that the line is inside the castle (which is cool) and it is somewhat loud in parts and it is DARK. My son didn't want to go into the castle on our first visit, so even if we had had two adults to ride it (right now only I can ride it because of the restraints and DH's size) it wouldn't have worked.
If you think he can go in, I would recommend earplugs just in case the sounds get to him. And don't be surprised if he needs some assurance in the darker parts of the castle. I probably wouldn't make that ride be his first experience with their rider swap system, so that he KNOWS that there's a good spot for him and a parent to wait while the others ride.
Hope that helps!