How does he feel about going to school? Is he excited and looking forward to it?
My 4-year-old was always what they call a “divergent thinker,” intrinsically motivated and not particularly susceptible to peer pressure, social pressure, rewards and punishments, or any of the usual external motivators. One result was that he was perfectly fine with using his Pull-ups at his convenience and leaving us to clean him up. I was very sure he was able to exert control but he just didn’t feel like it. (Bringing this one up was not easy.)
Even though he didn’t care about stickers and treats, praise, promises, etc., he did want to go to school. I said, “You know, you can’t go to school unless you can use the potty like a big boy.” He said “Oh, okay,” and the next day he was using the bathroom properly. There was never any further issue about it.
I don’t know about your little guy, and it does sound like there might be other issues for him, but I know that if we’d tried to go the psych route with our son, he’d have known right away that he was being handled and would have turned it into a contest, with him holding the aces. Who knows how long it would have taken then, or at what cost?