Have you tried a social story with him? Is he verbal? If he's not, does he have a PECS in place to help him when he needs to go. In short, if he freaks out when you set him on the potty, he isn't ready.
Many children with autism, especially ones that aren't high functioning, tend to potty train much later. My son was 6 when we were finally able to train him - and that was a HUGE day. Of course, once he did it, he has never ever had an accident, he's now 13. I made it very visual for him, put it on his schedule several times/day, and gave him a reward when he went (honeybuns were his favorite, fortunately at the time, a neighbor stocked vending machines and would let us have them at wholesale!). Good luck!
at 8:42 AM on Apr. 28, 2011