You could take a small piece of his poop and leave it on the pad and see if that gets his attention. Or just rub some of it on the puppy pad.
I have a little one who poops on the floor (but pees on the pad)... but since he is in the exercise pen with two others... it seems like he MAY be catching on from them that the poop belongs on the pad... (not next to the food/water bowls!!
).
Good luck!