J.O.
Hi S.-
I'm so sorry that you are experiencing this, but I'm even more sympathetic to your son. First of all, I would probably agree that something at school, whether it's as benign as being distracted to the point of missing the cues or whether something is happening at school that is triggering this regression.
That the school would blame you or not take your word is ridiculous. I'm sure the school is just concerned that some other issue may be going on at home for this to occur, but if he's not having accidents there, I'm sure that's not the case. Frankly, I would ask your doctor discuss the issue with the school nurse to clarify the situation. This way, they will have an outside, unbiased opinion on why this is happening.
As for releasing his medical records, don't do it. For them to even ask is crossing the line. Keep those matters private, it's none of their business what viruses, rashes, etc. he's been treated for.
If none of this helps, I would definitely consider switching schools. Your son should not feel bad for having a medical condition, nor should his teacher, principal, counselor, or nurse make you feel responsible.
I hope this helped a bit... good luck.