When my son started kindergarten, he had the same issue, but it wasn't obvious because he was at school. He ended up with encoperisis and is now doing fine.
There is so much going on that they don't want to stop, and they don't want to get poop on their hands, etc. I think they kind of become aware of the body function and aren't 100% comfortable with it.
The pooping schedule really does work. 10-15 minutes after dinner have him sit on the potty for at least 10 minutes with a book or whatever will hold his attention. He will be relaxed and he'll poop. And it'll get him into a pooping routine. Good luck.