Yes my answer assumed a puppy, but the getting near the door means being taken out should work, perhaps slower with an older dog.

Now that you have the clue of the tanking up, take him back out before he relieves himself. Walk around using a leash if you need to. When he urinates, lavish praise on him.

Much of your success may depend on his past experience. If a dog has been living where it could keep its living space clean, it should quickly catch on. The important part will be teaching it that if it goes to the door, you will let it out. It will be much more difficult if the dog was forced to live in its filth. You will need to learn to read the dog and learn its schedule, and when it needs to go out. Keep it in sight, closing doors and setting up gates.

Thank you Labman.....yes, we have been doing all of that, and that is why he is now mostly clean indoors. As you say though, it is the getting him to go to the door, or ask in some way that is the problem.