My Son has SPD, ADHD and Hypotonia. He is 3 1/2

My son WILL NOT use the potty. No way, no how, the only times I've gotten him to go is when I have him in underwear all day, he holds it in, and he starts dancing around grabbing his crotch and I grab him and hold him on the potty until he goes. He is the only kid in his class who isn't trained and if he is not trained by the fall, I am going to have to find a new pre-school. People make nasty comments and my OT's suggestions don't help. I've tried every single method. Any ideas?