S.W.
I believe that if you save bribery for the really big challenges, then it's okay to use rewards for something that is difficult for him. Research has proven that external incentives are best when ONLY used for particularly difficult or new tasks. So I might suggest offering a slightly bigger reward for going poop on the potty. I am also a fan of taking a day or two and leaving him naked or in underpants, not going anywhere, and taking him to sit on the potty about every 20 minutes. It's a long day, but it usually gets done fast. Also, if he ever pees sitting on the potty, then he might just end up going poop one time, and then he'll see it's not a big deal, and then if he gets an extra special reward for it, he'll want to do it again.
If that doesn't do it, I may be inclined to think he's not quite ready--especially if he's fine pooping in his pants. In that case I would likely wait a little longer and take it up again in a couple months.
Hope that helps. Best of luck.