Your issue sounds alot like my younger sister's issue with her middle son. I have a 2 year old nephew, which would poopie in his pull ups all the time. My younger sister will not take that risk, to put "big boy undies" on him. No matter what, he is in pull ups at all times. I got some advice for you, if this keeps up, put him in pull ups or diapers at all times. He may be may be scared to go poopie in the potty.
My son who is almost 3 refuses to poop in the potty! He prefers to poop his pants and also sometimes asks for a diaper to poop in. He is fine peeing in the potty and will even do so himself, running to the potty, pulling up his stool and lifting the lid so he can do it "all be himself". He just will not poop in the potty and is fine to go around with poop in his pants. If he sees poop it makes him gag and almost throw up, so how can he stand walking around with it in his pants? We have tried bribing him, sitting for hour on the potty, reading books, offering new toys, stickers, etc..... I don't know what else to do? Any suggestions?