Also-- CONSULT with an outside professional and get buy-in first, but then--

I'd withhold attendance until they have your IEP/504 meeting. Period.

Calmly inform the school (at the bottom of your letter of understanding would be ideal here) that you are, on the advice of Dr. Expert, keeping your son OUT of an undifferentiated setting which lacks basic accommodations for his disabilities.

Then-- DO NOT NOT NOT discuss accommodations with anyone again until they find him eligible. Trust me.

Schrödinger's cat walks into a bar. And doesn't.