The only solution I can see is to leash him to you. That way you can catch him at the moment he tries to mark. This is a pain for you---but the only way you can be sure he won't mark and the only way you can correct him at the right time. If he dislikes being leashed all of the time when he's in the house (but not outside), it might sink in. Don't expect immediate results. Give it a few days at least.