J.S.
Did you try rewards first? Like every day he doesn't have an accident he gets a sticker or something small? Maybe talk to the teacher to see if she will fill him in on what he missed while in the bathroom -- or let another kid tell him anything that happened while he was gone -- so he doesn't hold it. I don't know if that would work for his age. You could try practicing at home -- you pretend to be the teacher and he asks for the potty and you praise him and act happy that he asked.