We just got back--my son is currently diagnosed with ADHD, SDI, GAD, and NVLD. I agree with bookwormde--the asperger's/ASD threads were very helpful to me.
One thing we did this year now that he's a little older is that when we showed the GAC to the CM, we asked him or her to describe the ride, and to tell us if there was anything scary. It varied A LOT what he got upset about, and mostly it had to do with unexpected things. Splash Mountain was his favorite ride last time, but this year he got really upset if water sprayed him so he moved on to BTMRR. Another kid told him Stitch was great, so we talked to the CM (because I was worried he'd FREAK), and asked her to describe it. She told us water sprays on you, and he had the chance to think about whether he wanted to go on the ride or not. He decided to do it and enjoyed the ride a lot, much to my surprise. Asking CMs about what the ride was like worked for my NT daughter as well, btw, and with one small exception, the CMs were eager to help and had a good sense of what might be scary or hard for a kid on the ride where they were working.
One other thought is of course to make sure (to the best extent you can) that he gets enough sleep, that he gets the breaks he needs, that he gets into a cool place once in a while, that he doesn't get too hungry...all that stuff that keeps us afloat, but even more so at Disney. Also, at a couple of parks we had a touchstone place or ride that was familiar and I knew he enjoyed. We could go back there to kind of "reset" things as needed when they started to go bad. The GAC made a huge difference in our ability to be flexible.
Cate