lademps,
I just wanted to give you some hope that your Disney trip will probably be a positive experience. When my daughter (since diagnosed with high functioning autism) was 2 1/2 we took a Disney trip. I was really worried about the crowds because at this point in time, she would scream when strangers looked at her or talked to her (such as nice old ladies in the grocery store). I thought it might be a disaster, but my husband's company was paying for the Yacht Club, so how could we not go?!
We did get a GAC, which at the time, amounted to an automatic fast-pass (this may have changed). I was really worried, since it still meant standing in line with lots of strangers, as opposed to having a separate waiting area which is what I had hoped for. Anyway, it turned out fine. Since no one in line looked at or tried to talk to my daughter, she didn't seem to be affected by all those people. We also made sure that she was generally in the middle of our own group. She loved all the rides and pretty much everything about Disney. Her very favorite was the electric light parade.
Also a note about characters that may or may not apply to other ASD kids: We thought my daughter would be thrilled to meet Minnie Mouse, since she's always been a big fan, but she was scared of Minnie and most of the other characters. The only ones she finally warmed up to were Chip, Dale, and Pluto. I'm not sure, but I suspect this is because they look a lot less "human" (i.e. not wearing human clothes) that some of the others, so it was more like petting a dog than meeting a person.
And last of all, it was during this Disney trip that my daughter said her own name for the first time ever! I know I've mentioned that on these boards before, but it was certainly our most magical Disney moment!
Have fun and just experiment around to find out what your son will enjoy most. And keep up the positive attitude!
snoozn