potty training

how do you help your little one with withholding? (English isn’t my first language, bare with me 🥲) so basically my son has been withholding poop since he was 6 months old. We already changed his diet, he goes out for two hours we try running and playing with him so he’s more active, he doss drink water but not that much just 8 onz. We try giving him prebiotics+ fiber in his orange juice and if needed miralax. We went twice to different gastroenterologists and they all say he is fine its more of a psychological problem. My son is autistic. he does well with peep and loves the restroom like washing hands, flush toilet, toilet paper all of the routine. but every time he needs to poop and i know its soft he gets scared and tries to hide. we try reading books, cartoons. If anyone can message me or share what helped you that would be amazing. Thank you so much 🤝💞