I have no legal advice to offer (sorry) but I have heartfelt empathy. SPD was something we were looking at for DS8 (later diagnosed with ADHD & language processing disorder) and pooping "normally" was his nemesis.
Just like your son, he wouldn't feel it until it was just about to happen, or even until it was already in his pants. Preschool (4) was a risky game (never had an accident, phew, but he was only there for four hours per week), Kindergarten (5) was tricky... again, no accidents but I sent him in a pull up twice and kept him home a couple of times because I sensed something was up. Grade 1 (6) was the first time he actually had an accident in school. (sigh).
It was the last time it happened, and now he's 8 and going into grade 3, and it all seems to be resolved. No more accidents, no more smudges.
Anyway, what I wanted to say was that for his preschool (and onwards as well) they insisted on total potty training. The days when I sent him in pull ups were unknown to them. I felt justified in breaking the rules because the system can be so ridiculous sometimes and unfair to kids like ours.
Hang in there
I hope you get the legal leverage you need.
