4 YO autistic son potty problems

So my son is going to be four years old in about a week. He is autistic. He is not low functioning by any means. Most people who meet him don’t know he’s autistic, so I don’t think the issue we are having is related to an inability to potty appropriately like some lower functioning autistic children. I think it’s more of a behavior/fear/lazy thing? We have had him potty trained since he was about 2 which is actually earlier than my other “neurotypical” children. Anyways these past few months he has been holding his bowel movements four days up to almost 2 weeks. He will let tiny little bits out at a time in his undies, and it is an absolute mess. I have tried setting him on the potty every 30 minutes making him sit there while I read him stories etc. he will not go poop on the potty. I’ve tried leaving him alone on the potty. I’ve tried everything google has suggested I feel like, yet this trend continues. His pediatrician doesn’t see a problem, but his therapists and I do. He will randomly go poop on the potty without me even knowing I just happen to go see an unflushed bowel movement in his toilet, and it’s definitely his because they don’t flush because of the size. Like I’m sure that super gross, but he holds his bowel movements in for so long they build up into these huge things I don’t even know how he pushed out. The pooping in his undies has got to stop! It’s disgusting, and I can’t understand why he does it. I’m worried about his health too because he holds it in for SO long! Has anyone else dealt with this? What did you do?