C.C.
Have you tried leaving him alone on the potty instead of standing there watching him? My boy is entering potty training age (I hope!) and my co-worker said that giving him some control and privacy may work (as it worked for her child). She said that when she was in the bathroom, her child wouldn't do anything, and then would poop-on-self 10 minutes later. When left alone (I'll be back in a little while, call me if you need me) in the bathroom (but in the next room and listening) her child found consistent poop success.
Good luck!