A.J.
My son was the same way. He would actually have me put a diaper on him and go hide when he needed to poop! The Dr. told me that it's sometimes very scarey for a boy, because they have things already 'hanging' down there. So, when something comes out of them and goes into the potty, it scares them to death.
It went on for awhile. Maybe a few months, before he would start going to the potty for this. So, just give it some time. He'll let you know when he's ready.