Potty Training

We have been potty training Trey since July. He is doing great, except for pooping. It does not bother him at all to poop in his underwear. We have tried everything to get him to poop on the potty. We have gone as far as buying Murdoch the train from Thomas the Train and placing it on the shelf above the toilet for a little encouragement. Today I was trying to get him ready to go to the hospital to see my Dad. I called his name and he didn't answer, so I went to look for him. My first stop was the bathroom. There he was....
shaking his underwear like crazy above the toilet trying to get the poop out of the underwear and into the toilet. (He has seen me do this way to many times) Needless to say not all of it got in the toilet. I took the underwear from him and sat him on the potty to finish up. He was able to finish the job on the potty. When he got up he said, "Now I get the train!" He doesn't seem to understand that pooping in his underwear and then shaking it in the potty doesn't count for pooping on the potty.