It sounds like you have tried it all. Just wondering if you have tried not putting any underwear, diapers, or anything on him right after he eats until he goes to the potty? Would he go in the floor, or what would he do if you did that? I've been a professional nanny for many families and I have seen families do this with children who just would not go to the potty for bowel movements and it worked. They don't like it, but when they have to go, they go to the potty because they don't have a diaper to go in.
All the best to you!