Sounds like he's hard headed. I would probably keep it up--or make him stay outside most of the time. If you are taking him out at set intervals, he should get the message. Don't wait for him to tell you he needs to go. Set up a schedule and stick to it. When he is inside, he can be crated or tied to you.

If that doesn't work, I don't know what will. Maybe Nancy Efusy, pet psychic, can help. She got my old dog, Samson to stop watering my artificial plants and the dining room table legs. It was a worthwhile expenditure. Even my vet couldn't help.

Let us know of your progress.