A.M.
My daughter did this for a while. She was trained by 2 and a half and then at maybe 2 and 3/4 she started the poop in pants. Luckily, she LOVES icecream and so I told her if she would poop in the potty, she would get icecream. After a few weeks, she got back to pooping on the potty. Pooping seems to be quite different from peeing for kids. I am not sure why. Is there something that you know he would love, that he can have each day if he poops in the right place? good luck.