Poop just seems to take a little more time to learn where it goes for some reason, lol.
I know this is gross,

but take a little piece of poo and put it on the pee pad, and when he goes to smell it (which they ALWAYS do, little stinkers) say GOOD DOG, poop on pad! all excited, or something to that effect.
Patience is a virtue, and I know me, I am not always that virtuous, lol!!