Here lies the problem. The actual seizure itself can cause your child to fall, flail and injure himself. The school does not want to be responsible for any lack of medical attention that your child may need.
I have no idea what medications your son is taking for his epilepsy, but if he continues to have seizures, then it would appear as though his medication is not effective. No, I am not saying you are not caring for your child properly, so please do not misunderstand my comment.
Unfortunately, there are so many law suit happy people who try to make money by suing people, that it has forced establishments to take counter active measures. Yes, the ambulance and hospital bills are very high and damn near financial extortion for medical attention. I get it. But, you simply cannot ask a school to ignore the fact that your child has had an epileptic seizure or expect them to assume he does not need medical attention. What would happen if he did have a seizure and he bumped his head? Should the school take that chance and not seek medical attention? Probably not.