Hi S.,
Almost all the kids at my son's preschool (including my son) did something similar at that age. With our son, we had been using a sticker chart for potty training, and he got a special balloon sticker only when he pooped in the potty. It seems like kids think they are losing a limb in the toilet or something! Also, many people I know bought a small but much desired item and put it in the bathroom, within eyeshot of the toilet, and let the child have it after the first poop in the potty. We also spoke to our son about relaxing (made a silly looking relaxed face and said "blaaaaaaaaah") and that holding in his poop leads to pooping hurting. Our son had suffered from constipation, and that made him avoid the pain of pooping. We explained that holding in poop made it worse. It took a month or so of balloon stickers, but he moved on from that stage. We also tried not to sound super concerned because that leads to a total power struggle (had already been there, done that, and had to take a several month break from training because of it). My cousin had a similar issue w/ her son, and her pediatrician told her to use a Baby Fleet (or a half an adult Fleet) enema. We had to use them a couple of times when our son was younger to relieve his constipation pain. They cause some cramping and then pooping within a minute, and I promise you your son will not be able to stop it. I don't know quite how I feel about this approach personally, but my cousin followed it. She gave her son the enema, put him right on the toilet, and gave him a special treat right after he went. For her son, going once or twice in the toilet and seeing that he was okay seemed to take care of the problem. This is TMI (apologies in advance), but before I could bring myself to give my son (about 18 months at the time) an enema for his constipation, I had to know what it would do to him, so I gave one to myself. It gave me some cramping, which was uncomfortable but not really painful, and then, well, I think you can guess. It was all over quickly and I felt fine afterwards. Just remember that you will get through this (eventhough, if you're like me, you've prolly convinced yourself that your child will still need a diaper to poop when he's 12!). K.