won't do bm's on the potty

[mom of two]

my son just turned 3 last week. he has been wearing underwear all day and night for a month now with no accidents. the issue i am having is that he refuses to poop on the potty. he will insist on a pull up when he has to go. yesterday he pooped his undies for the first time. every time he has to go i ask him if he wants to go on the potty and he says no. after he goes in the pull up, we empty it into the potty and flush it away. he will be starting nursery school in next week and pull ups or trainers are not allowed. luckily he will be in my classroom (im a teacher) how can i get him to want to poop in the potty?

[Neera]

is he a constipated child? we've had the same issue and haven't had any success for a really long time. have tried just about everything, but dd was severly constipated and bloated. i decided to get rid of all diapers, pull ups last month and we had wailing and screaming sessions. so, she is back in pull ups on her doctor's recommendation and because I couldn't fight it any more. here's the link to my post on the issue:

http://www.mothering.com/discussions...207&highlight=