Does he have the communication skills yet to ask him if it HURTS to go potty? If he has a urinary tract infection or something like that, he might be afraid to go.
Either that or (and I hate to say this) it sounds like someone has scared the bejeebers out of this little guy. As in; "if you pee/poop your pants, BAD things will happen!!". A friend of mine used to tell her kids odd things like, "If you have an accident, the dinosaurs will come and eat you." Needless to say, it had a tendency to backfire on her. If this has happened, he may have developed some serious mental issues with toileting. Honestly, I would put him back into diapers and try this again a few months from now.