I wanted to assure you -- I teach kindergarten and while accidents are thankfully not a daily occurence, they do happen. I wouldn't be too worried about a couple of accidents in the last few months.
That said, I have a couple of suggestions. No belts - then he wont have to worry about it. Tell him to go before school and at recess, even if he doesn't feel the urge. Enlist the teacher's help -- what is her policy for bathroom trips (i.e. is it any time, they get up and go, or are there times that she needs the whole class together; do they have to ask before they go or not, etc....maybe he is embarrassed to ask.)
Whatever the circumstances, don't get mad at him. He is already distressed and embarrased enough about having an accident at school with his peers around. He needs to know it is OK, it happens, and you'll just work to help him do better next time