I always put my two outside together if not for convenience on my part but they usually run around together outside too.

I've also trained them to potty on command. When I let them out I watch them. THE SECOND they start going potty, I say "GOOD POTTY!! Good potty girl!!" etc. Now I'm trying to help them distinguish between going outside to potty and going outside to play. If they're going out to potty I say "let's potty!" and if they're going out to play I say "let's go outside!!".

Teaching them "potty" also helps if you're on a walk or in apartments to make them potty on a leash.

Anyway just give your pup TONS of praise for going potty outside/on the pad and absolutely NONE for going in the house. Eventually he'll get it, "hey if I potty on the pad mom is REALLY HAPPY and gives me lots of attention but if I don't I don't get any attention at all!" and he'll learn in time!!