I think incorporating some crate training if none is being used will help. If you take him outside and he does not go potty, when you bring him back in, he must go in crate for about twenty minutes or so and not be allowed to roam or play. Then outside again to potty spot. If he does not go again repeat in crate for twenty minutes then out to potty spot and naturally praise like crazy when he does go. The idea is to get him to go potty when you want and where you want. I felt using the crate was extremely beneficial.
Chris Battistelli