Get some kennel fresh or some other safe type of disinfectant that will take out all the smell of what he already has done and what you can smell of the other dog that was there.. then go and get a yellow stake/puppy piddle post/pheromone stake from a pet store or online is cheeper and put that outside where you do want him to go.. and that should work .. if not keep me informed I have many other ideas for you I had the same problem with my Chihuahua.

as for one person suggested it being a medical problem you can easily tell if he is lifting his leg and peeing it is not .. if he is jumping up and happens to pee it is..