J.W.
Do yourself and him a favor and stop trying until he is ready! My son was not potty trained until he was over four years old. They just don't get it but when they do it is easy.
My son was in the programs since he was three and yeah they would have loved for him to be potty trained. Strange really you would think if anyone understood it wasn't going to happen it would be the school that deals with these kids all the time.