I got excellent advice from the Autism center down in Gainesville, Florida (called CARD-it's state funded and free). They set me up with a program that they've used 100's of times and it worked for us, too. We had same issues as you. Look them up on the internet and see if they can't help you locate a similar program in your area. I attended a potty training seminar and used the schedule they created and it worked. One thing they said was that we had to break things down into tiny steps. First, child has to learn to know he has to go, then to learn to grasp pants waistband. Next, pull them down, next sit comfortably on toilet, then wipe, etc. You just can't say, "Well, time to learn to use the toilet!" because the things that you or an average child would take for granted, like taking your pants off, might be not even on the radar for the ASD child. So you say, "Go potty!" and he wonders, "How am I going to do that with these pants on?"
We bought DS really lose fitting pants with easy waistbands because he often will still not realize he has to use the bathroom until the very last minute, and he's been trained for two years now. Even now, the bathroom can be an issue. He still does not regularly wake up at night to use the toilet. Two months ago, he was on his way to the bathroom at school (he's in 1st grade) and a teacher saw him walking down the hall and old him to go take a test in the computer lab, so he did. He did not tell the teacher, "I was on my way to the bathroom!" because he was afraid of making the teacher mad, so he wet his pants. He was very upset and embarrassed.
This is all a result of what they call poor "executive function" so you have to constantly create a script in your child's mind to help them remember the steps each time they have to do something. Some kids like pictures, or charts, but first you have to master each tiny task and not throw the whole process at them and expect them not to be completely overwhelmed. You wouldn't expect them to figure out a Quadratic Equation on the first try; you'd teach them basic math and work your way up to harder things. Keep this principle in mind as your child gets older, and life will be much easier.
