I know he must be a sweetie. The only other suggestion I would have is to get a black light and look at your carpets for stains. You will probably be shocked! I would then consult and hire a company to come in and clean it thoroughly with a pet stain removal special cleaning.
It sounds like you take him out, but did you train him to go to the door and ask to go out when he needs to potty? That's about all I can think of. There has to be a breakthrough with him. They are too smart not to understand.
Also, you could put bellybands on him when you let him in the rest of house.
