Absolutely agree with Sandman, children don't at the best of times like to ask to go, they would sit and wet themselves first ,ask teacher to be discreet in the reminder .The bullying would have made it worse ,I hope you got that ironed' out with the school .
My experience as an elementary school teacher and principal is that if a kid is being bullied, that last place he will go during any break is the school bathroom. He may have a teacher that (hard to believe) doesn't let them go during class or makes a big deal of it. So he tries to hold it in all day.
You need to talk to the teacher and let her know that it is extremely important to let your son go to the bathroom during classtime and not to make a big deal of it. Then let him know that it is ok to do so. I would also make sure that the teacher and the principal know about being bullied. There is absolutely no excuse for that happening - not to mention laws that protect the child and punish the school for not dealing with it. Hope this helps!