S.O.
Put him on the toilet about the time that he would normalle poop. Put him on during a poop - take him, take off the diaper carefully and put him on to finish. Put him on after he poops in his diaper, take it off carefully - as he's standing with legs apart and put him on the toilet anyway. Wipe him mostly on the toilet - and then finish if need be with him off, standing, legs apart. He will get the idea eventually.