My Son Has A Medical Condition And His School Refuses To Accommodate It

My 3 year old has been potty trained for months. He started preschool and they have a “potty agreement” where if the student has accidents 3 times within a week, they need to stay home for a week. My son has been peeing more often and had accidents Monday and Tuesday. I took him to the doctor today and they diagnosed him with overactive bladder disorder. His bladder muscles cannot hold his pee, and it can happen because of growth spurts. It typically happens, the doctor said, with kids who had abdominal surgery (he had hernia repair at 2months and she said that can cause it) and is a temporary thing but can last upwards of 8 months at worst. I got the documentation from his doctor and took it to the school, and they are refusing to accommodate him or give him leeway with the stupid potty agreement even though he cannot physically help it. He had another accident today (mind you, these are the first accidents he’s had in weeks, and even when he did have accidents before it was like once a week. This is NOT normal, thus the diagnosis). Is this legal, them not giving accommodations for his medical condition? It is a Montessori preschool in Ohio. The school (private PK-12 school) gives medical accommodations to other students but not mine. I am just at a loss. I need advice.