T.T.
We just went through this with our 3 year old. What I ended up doing was buy him "poopy prizes", a mixture of toys (most from the $1 bin, but two big ones) I then showed him the prizes and told him that they were for when he pooped in the potty. After that, every time he would poop in his underpants I would act very disappointed that he won't be able to pick out a prize. Sometimes he asked to see the prizes, which I let him, but he could just look. It took about two weeks, but he finally pooped in the potty. I then gave him one of the big prizes. After that first time I would let him pick one of the small prizes every time he pooped. When all the little prizes were gone we considered him potty trained and gave him the "grand prize" which was the other big toy (a toy he's been wanting for a while) I warn you, he had about 2 days where he went 2-3 times a day for a prize, but it was enough to get him over the hump. He's been using the potty every since, with NO accidents!!
P.S. The first time he went was the first time we put him on the potty backwards. He loves it, I think he feels secure that way!!