I'm not sure if you've tried this or not, but sometimes it works if you get him some special underpants to wear only at night. Not nappies, but "big boy" underpants. If there is a specific character that he really likes, he may be discouraged from pooping on them. Otherwise, you may want to consider changing his diet to something with more fibre so that he can't hold it until nighttime. Put a lot of emphasis on how big a boy he is, and how big boys poop in the toilet. Take him to school yards and let him see other kids at school and encourage him to be like them - reminding him that kids that go to school aren't allowed to if they wear nappies.
Don't do it as a shame technique, rather encourage him to be a big boy, and provide him with a special privilege if he does poop in the toilet.
