My 3 1/2 year old won't poop in the potty!!?

he has been pee potty trained and been wearing underwear for a year now, but he will not poop in the potty! he doesn't pee the bed, pee his undies, or anything. he just goes to the bathroom and pees when he needs to. when he has to poop he asks for a diaper. we give him one to poop and put his undies back on... show more he has been pee potty trained and been wearing underwear for a year now, but he will not poop in the potty! he doesn't pee the bed, pee his undies, or anything. he just goes to the bathroom and pees when he needs to. when he has to poop he asks for a diaper. we give him one to poop and put his undies back on him when he's done. he never poops his pants. we try putting him on the potty when he tells us he has to go, but he screams and will not go! we've even tryed putting a diaper on him and setting him on the potty and he won't do it. it's something about sitting on it he doesn't like, and won't tell us why. we've even tryed bribing him with everything, even disney world and monster jam and he still won't go. is there anyting we can do at all?
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