Third, I realize that you've said he has a confirmed medical condition, but children with other confirmed medical conditions aren't allowed to go to school either (e.g. infection with a stomach virus or other highly infectious diseases).
While this is true, those diseases do not qualify as disabilities, because they are temporary. Encopresis is a disability, so he should at least have the protections of the ADA. (I've thought about this a lot, because my DS4 is not potty trained either, and may have the same condition.)
I still don't think this necessarily qualifies for a full-scale attack mode, certainly not yet. After all, if you do get him in, you want to be able to work with the school. I would probably start by writing them a letter explaining that while you understand that being potty trained is normally a condition for being allowed to enroll, your son has a disability that interferes with his potty training. Perhaps point out that if the problem was that he couldn't walk, they would put in ramps for his wheelchair where needed, not refuse to allow him in the door. Explain the plan that you have to mitigate any dangers to other students and to protect him, including contingencies for the things that Val mentioned.
I suspect that a pleasant letter that explicitly uses the word "disability" will cause them to think twice.
