I agree with babygirl....This is what I would have said to do!
some children have a thick bladder wall which makes it easy to ignore the bladder until it is completely full- actually I believe they cannot feel it- so busy children of course would have to run for the toilet at the last minute. They can only feel the fullness when it is too late. Perhaps he was being taken to the rr more often in those early days by you and you got him to empty his bladder before it filled.... SO he just needs to understand he has to TRY at the times when you and caregivers offer, and offer every two hours and always after meals and snacks, as well as before bedtime and first thing in the morning. We have a 2 yr old at preschool that can hold it 11 hours. Her mom says she holds it for 8 hours at night then waits another 3 hours.
good luck
No pullups,at his age thats punishment,, no punishment, find out why, ask his teacher if the children have regular trips to the Bathroom, some children dont like to ask to go she should tell them they can go and make sure they do, a lot of teachers are amiss with this ,perhaps they dont realise small children need reminding, . Could there be anything else he would be upset about does he have friends at school, ask him how he likes it there .It could also be he forgets when hes busy till its too late, hes not alone this is a common problem.