I went to Disneyland when I was six, and Disney World when I was 8, so I've got your son bracketed.
While I loved the rides, especially ones that involved zooming around in the dark (ok, I'm really old and Space Mountain was really new), the whole experience was overwhelming and very difficult for me. There was Just. Too. Much. Going. On. All. The. Time! There were too many people, too many things to see, too many things to do. There were long lines, with too many people close to me. Too much noise, too much everything!
Heck, I won't even go to the mall. Disneyland is a very unpleasant thought. Maybe if there were a place that your son could have a bit of 'downtime' or quiet time, the experience would be manageable. But it's tough to find that at a theme park where the whole idea is to have a very active and stimulating "good time."