Autism parent…

Idk if this is the right group, but I have tried the autism groups and no one seems to have answers. My 8 year old is nonverbal, level 3. He is having potty issues. His ABA therapists say he is peeing his pants to get out of activities (he had been pee potty trained for years and this does seem like something he would do) but I was concerned so I took him to a GI specialist because I was concerned he had encopresis/a blockage. The doctor said all is well. The main problem is he has this kind of pooping ritual? And I want to get feedback from experts. He pees just fine in the potty, flushes and everything. But when it comes to pooping he will not do it. He will hold it until night time and poop in his brothers bed. We took away nappies but he will poop in underwear. He also smears. If we interrupt his process and take him to the bathroom, he will just stop and wait until we let him go back to his room and poop again in fresh underwear. We have tried visual timers, visual stories, using screen time, routines. I just don’t know what to do anymore. Has anyone had a kid like this in their care? His therapists don’t seem to know what to do either.