Hi, L. -
I have the same trouble with my 4 year old. He waits to poop and when I put his diaper on for overnight, suddenly he says he has to go poop and does it in his diaper. I am amazed that he has that control!
I think the trouble for us is (and this revelation was sparked by another mom's comment) that he learned to pee in the potty standing up. So sitting down on the potty is completely foreign to him. He's afraid he'll "get splashed" and all sorts of other phobias.
My older son learned to pee on the potty sitting down, so it was naturally easier for him to get the sitting to poop thing down.
So I may try having him sit down to pee so he gets used to sitting and see if that helps his fear.
I'm not sure if this is the same for you, but it's a thought. Otherwise, we have just tried to encourage him(threatening never works for my son) and we keep telling him he can do it. We've also tried bribes, but they don't really work on him either (but they may work for you).
I'll be reading the responses and follow up - good luck!
--M.