I wouldn't see this as a potty trained or not potty trained issue. Your kid has a medical issue. One that is not contagious. To me that is different. Plus it sounds like you are there to handle any issues that might arise because of his condition.
Is there a Children's hospital nearby? Maybe a social worker type person could give you some pointers. I am wondering how this would be handled if he had an ostomy. Of course it isn't exactly the same but it seems close enough that what might apply to one situation could apply to the other.
