Assistance Please!!!

Ok...we've been "potty-training" for almost 2 months now...Porter is GREAT at peeing in the potty. He rarely has an accident; normally his pullup is only changed after his nap. But, we have yet to get him to POOP in the potty. He yells, screams, and cries when we try to get him to "do his business" on his potty. Normally, he runs into the room with a diaper and asks one of us to put it on...shortly thereafter we're cleaning him up. I've tried sitting with him, reading books, hanging out in the bathroom...nothing! He'll be 3 at the end of September, so I'm not in a total panic yet. But, I would gladly appreciate any advice anyone has...I just don't want this experience to end up traumatizing him in the end!