I'm certain that he isn't the first kid in his class to do this. It happens. Most K teachers know this is embarrassing and will protect kids from teasing.
I would ask him if he thought the belt was a contributing factor. If not, shrug off the incident as "no big deal, happens to everyone" sort of a thing. If it was, suggest he not wear the belt until he is more confident with it. Either way, I wouldn't make a big deal about it at all.
Unless, of course, he reports that his teacher denied him restroom access, then I would take it up with the teacher. But don't borrow this headache unless it applies.