I have dealt with different parents and different kids on this issue - it has many variables but here's some of what's worked (depends on how strict you want to be and how "stubborn?" you child wants to be)
If he's asking for a diaper, make him poop in the diaper in the bathroom and eventually work up to pooing on the potty with the diaper on, and working to on the potty without diaper.
If you're doing reward system, find out what HE really wants(stickers, candy, chocolate, fruit snacks, toys, tv time, park time, pool) you can also do reverse reward system: take away ALL his toys (or all of something he covets like thomas trains) and he can pick back 1 toy to have when he poops on potty.
Go naked for a week (putting your life on hold for your child, go no where!!! Spend lots of time with him and look for cues when he needs to go and help him relax on potty with movies, books, games)
And I think my last suggestion is make him change his pants himself when he has an accident and put his dirty pants in the laundry room and put on his new clothes by himself. For some it is so frustrating to get undressed and dressed they give up and go potty soon, but you have to stick to this, even if it makes you late for something. My niece absolutely felt disgusted in wet jeans, so we put her in jeans everyday (they are also the most hard to get off the body)and she never had an accident again!