3.5 year old holds it all day

So my 3.5 year old loves wearing his big boy undies. We go try to use the potty every hour. He’s made it through the day with no accidents but he refuses to go in the toilet. He holds it until nap time or bed time (the only time we do pull ups). I’ve tried everything to get him to release and nothing is working. He chugs water all day long so it’s not a dehydration issue. He’s starting preschool tomorrow and I am really hoping the teachers get through to him. Anyone have any tips?