I've potty trained 3 kids, but my 4th is stubborn when it comes to pooping on the potty! He's been peeing on the potty for months and months - doesn't have accidents, goes overnight w/o accidents - but will NOT poop on the potty! I have done the same, bribed him w/ everything under the son. Lord help me there have been some serious acts of bribery in this house, lol.
But through it all, I know he'll do it when he's ready - it's killing me, I've been changing diapers (and poopy underwear) every day of my life for 11 years and 10 months straight - so I'm desperate for him to poop on the potty, but I know it will come one day :)
My 3 yr old goes and hides in a room and poops in his underwear and will tell me when he's done (geez). But one day, one day he'll decide he's tired of it and will go on the potty - that day can't come soon enough :)
My 3rd child had a hard time pooping on the potty also, he had some other issues though - so I gave him a little more leeway (food allergies that led to intenstinal problems) - so he would hold it and hold it and eventualy become so constipated that we had to give him a suppository. That lasted about 6 months - a struggle everyday - but he finally decided it wasn't worth it and now poops on the potty just fine :)
So I know, one day, Lord help me one day my 4th child will decide, oh yes, he is ready, and will poop on the potty, and one day, your son will decide too :)
Good luck, I'm pulling my hair out with you :)