I hate to tell you that the only time my son regressed in potty training was when he had bad experiences at daycare. For a child, a bad experience could be something as small as teasing from another child or scolding from a provider for having an accident. Check in with the school first.
Also, children love feeling like they have control and potty training is one area where they have complete control...not much you can do to change his mind without bungling the experience and making things worse. Try giving him more choices in other areas such as food, clothes, shoes, etc. If he feels he has some control in other areas and you keep encouraging...not pushing him to use the potty, he may decide to go back to it.
Also, due to his autism and developmental delays, he just may not be ready. Lots of children start potty training with gusto and then realize that it is work, that it interupts their play time and they aren't ready to do it. Be patient, keep encouraging him and reading him books or letting him watch dad use the potty.