I don't have a child with autism but I just watched my sister who has 2 kids with autism/aspergers and she said the trick is complete consistancy. She set up a timed routine where her kids got a reward every time they sat on the toilet and tried (an m&m), when they suceeded with pottying or pooping in the toilet they got a seperate prize (pooping= small toy// pee= lolli pop) so they could earn 2 prizes and she charted it so they could see their accomplishments. It took her a few months with each child. But she managed. I hope this is helpful.
at 1:26 PM on Jan. 13, 2010