A.P.
We also had the same problems last year with our son. We had a terrible trying convincing him to use the potty for pooping. He would hold the poop in for days, which made the job all the worse because then it hurt to actually go. We would know when he had to go because his face would get red, he would hide, and he would cry.
We ended up using a "pooping chart." At first, anytime he sat on the toilet and tried to poop he got a sticker. After 3 stickers, he got a small prize. We did this for awhile, and then stepped it up. Eventually we replaced the trying stickers with stickers of actually going. We also increased the length of time it took to get the prize. So a chart for pooping 3 times, 5 times, 7 times, 10 times. We also made the toys greater as he learned to go. (ie. the toy for going 3 times wasn't nearly as cool as the toy for going 10 times.) We let him pick out his new toy each week and then kept them on top of the fridge. More motivation to earn that new prize!
I made the charts on computer paper and bought different cheap stickers. It's a great way to reinforce the behavior you want and counting. (ie. Look you have pooped 2 times, only 2 more times and you get your new truck!)
I have a picture of my son with his last pooping chart- full of 10 stickers and a brand new trash truck. He was so excited and I'll never forget how happy we were to be finished!
Good luck! Hope this helps you some. :)