J.F.
I know exactly what you are going through. We have a 9 year old boy with Aspergers. When he was young and going through that stage we were just told over and over that he was a little behind and he would "catch up." We were told to just keep trying and he would get the hang of it. Well, for us he actually learned by watching his younger brother who was 20 months younger than him. So he didn't learn to potty train until he was about 4 1/2 years old. I wish I would have known then what I know now about him. I could have tried so many different things!
I found a book for you that should be pretty helpful. It's at amazon.com. It's called Toilet Training for Individuals with Autism and Related Disorders. It's by Maria Wheeler and it is $13.57. It has 4 1/2 out of 5 stars. I hope this helps you! J.