Maybe if he likes to go into his room to poo, you could put a trainer potty in his room. Don't push it, just put it in there where he can see it. Maybe that will help him get the idea. Then if he uses it, give him some special treat, like a sticker.
Another suggestion, if you know the time he's going to have a bowel movement, is you can get him on the potty a little earlier than that, and read books to him until he has to poo... have him already on the potty BEFORE he knows he needs to do it.
Good luck! Our 2 year old is starting to get the idea of peeing in the potty, but just like your son, won't use it for bowel movements.