Oh, one more thing.
You can get him used to a command like "Do you have to go potty?" to where he gets excited and knows to go to the door if he has to go.
Each time you go to take him out on his leash to potty, before opening the door, ask him "Do you have to go potty?" in a somewhat excited tone to where you see that little nub start waggin' or just some show of excitement. When he does, open the door and go to your spot right outside the door and stand there. As soon as he is finished, right as he's about to step away from the mess he just made tell him something to the effect of "Yay! You went potty!" and then "Come on".....and go back in the house and do your thing.
Some dogs I've treated when we get back into the house, others not. It really doesn't matter. Just you displaying how pleased you are is more than enough for most dogs.
Happy training!