A.H.
I had this problem too. I think some of it is that they don't quite recognize that they have to poop. It took a while but my son did get it. I would catch him starting and rush him in to finish on the potty. Now he tells me for both. Just don't push things, he will get it. Good luck.
also I did finally use a reward for pooping on the potty, for my son he loves chocolate, so when he went on the potty he got a small piece. After he started telling me he had to go I stopped offering the reward and he finally forgot about it and now just goes.