What The Poop

Why won't Joey poop on the damn potty already? I'm getting so irritated. He's got the rest of it down: very few accidents, waking up dry, telling us when he needs to go, enjoying his 2 m and m's reward, but he flat out refuses to poop.

I KNOW he needs to go. How do I know? We won't get into it. I think the answer to that question teeters on the edge of too much information. So just take my word for it. I've tried lots of different approaches to get him to cooperate, and to no avail. I even pretended that his baby sister pooped on the potty this morning and rewarded her with a treasured popsicle. When he wanted a popsicle too, I told him he could have one as soon as he POTP. (pooped on the potty) That didn't even work.

Ok, so does anyone have any suggestions for this situation? Please?