We sent our HFA son on the pirate cruise when he was five. I was nervous as can be and I was sure that I was making a huge mistake but we sent him on. Now at first, he was excited and loved getting the Mickey Pirate hat. When he got on the boat however, he did start to cry. The husband, younger son and I toured around the GF while he was on his adventure. When they came back, all singing, he was all smiles (the CM did tell me he cried for about the first 10-15 minutes) but that he had fun. He talked about his "treasure" for the rest of the trip and was very talkative about it. I think it was a good experience for both of us. Now, had I waited another year, he probably wouldn't have cried and gotten upset, but that is an individual thing. If your son is good about being seperated from you and loves pirates than I say go for it. They have had plenty of autistic children but I would also tell you to get in contact with Guest Services and find the number to the person who runs the pirate cruise and talk with them just to make sure that they know your coming. We did and they were anticipating Payton and any issues he might have, they even assigned him a "buddy" (while the buddy was not soley there for him and was responsible for many other children).
Let me know if I can be of any help