Hi R.,
My just-turned-three-year old is experiencing the same thing. He has been going pee in the potty for a couple of months, but after spending an agonizing hour and a half with him in the bathroom to go poop one time, I decided to lay off that part. So for a couple of months, I was content with him being pee potty trained. He would either ask for a pull-up or I would put him in a diaper when it was time to nap, and that is usually when he would poop. Now, however, we have begun cutting out naptime, and he is in his underwear all day. This has prompted some success with going poop in the potty. The first two days were horrible - screaming and crying for about 45 minutes before he finally allowed himself to poop. After that, it seemed to get better. Then we had about a week where he seemed to hold it until nighttime, when he had a diaper on. The past couple of days, we have been back to using the potty. I second all of the others who have mentioned the different foods to offer (and not offer). I have been paying more attention to fiber content now, as I do not want it to be painful for him. One more thing - I tried all kinds of ways to make potty time fun (books, videos, sticker charts), and it seems when he was truly ready, we didn't need all that stuff. I wish you lots of luck.