My youngest daughter isn't on the spectrum but was terrified of the toilet. When I started the potty training with her I purchased a potty chair and just put it in front of the television. I had her just sit on it fully clothed until she relaxed. Once she was relaxed we then progessed to having her sit there trying to go potty (still in front of the television while a movie or PBS was distracting her) When she was able to finally go in the potty we then moved it into the bathroom. It took a couple of weeks from start to finish but it was worth it.

Now with my ASD son, we put him on the potty every 30 minutes and had him sit there looking at his potty book (that I made using pics of him doing all that he was supposed to from turning on the light through drying hands and turning off the light) for about 5 to 8 minutes each time. We did this for about a week, after that it was just staying with him (like white on rice as the saying goes lol) to catch him before he decided it was okay to hide and pee in his pants. We're still working on going #2 but we've got peeing down pat. :)

Good Luck!