My son is the same way - he will not use the restrooms at school and he is in the 9th grade. He will wait until he gets home - which can cause a problem if we have errands to run after school and he is delayed.
When he was in grade school he had a couple of tummy ache episodes also. Then the school nurse told him to come to her office as she had a private bathroom. That helped alot!
Does your son's school have a private bathroom he can try? That may eliminate some of the noise and stress levels. I also agree with trying to have him "train" his body and adjust the time he normally goes. But if he is sick maybe he could, with therapy, adjust to a more private setting if the school can accommodate that.
Good luck and God Bless!!!!