Is there any kind of cloth texture he does like? I was just thinking maybe you could fashion a toilet seat "cover" that he would be okay sitting on. Like sew a few cloth diapers to make a "cover" or something like that. Something that he likes and of course is washable since you'd have to remove it to clean.

And I know this might be really gross or "off" to consider, but maybe get some of those "potty pads" they use for puppy training and let him squat on one of those in the bathroom? Just to avoid the whole poopy diaper mess and the battle of the toilet. And maybe after he is used to that and likes going, he can advance to the toilet. I know...very unorthodox. Just trying to think of something to work with his sensory things.

Lastly....hang in there! My little ones didn't truly potty train til just before they were 4 years old. No sensory stuff, just not ready.