I just read your request, and I know it's been about a month, but if you're still looking for ideas, I thought I'd share one I read about recently.
First, I'm sure you understand that it's not an issue of doing something bad, like getting into forbidden cupboards or hitting would be. It's just a part of growing up. However, the fact that he can control where he poos tells me that he could do it in the potty if he wanted to.
If he goes in the pull-up, it takes more cleanup than if he went in the potty, right? Well, the families I read about told the boy that since he had gotten so big, he couldn't be cleaned up like a baby anymore. They would take him outside and hose him down with the garden hose. Since the water was cold and this new method of clean-up was undesirable, the little boy decided it would be better to go in the potty and was trained within a week. In the second case, the little fellow was so Spartan he decided to just grin and bear it, so they had to come up with something else. They told him that until he was old enough to go in the potty, he wasn't old enough to eat sugar. After a week of watching his brothers and sisters eat sugar--granted, not tons of it, but something like the spoonful of powdered sugar on french toast, a little honey in the oatmeal, that kind of thing--he decided he was big enough and that was the end of "diaper dumping".
Now, I know it's the height of summer down here, so hosing outside might not be as effective as up north--what with hoses full of warm water and if you live in the city limits you might not want to spend that much water--but maybe the stories will give you an idea of something you can do to encourage him to go in the potty. Without physically hurting him (even if you believe in spanking in it's place, it's not something that should be associated with potty training!), see if you can come up with something that is uncomfortable to him--even if it's just a mental discomfort. You might pick something he's fond of and tell him (in a very philosophical way) that you have decided that he's not big enough to do that, and when he is old enough to poo in the potty, then he will be old enough to do that. It just might help. :)
Good luck, let us know what happens.