The vet's positive it's not a medical problem? If it's definitely not a medical problem, I would go back to basics and train him like you would train a puppy. Make the crate his "den".. even if your dad thinks he'll be fine one night, don't give in. All it takes is one mistake to set you back in training.

I suggest keeping him by your side, preferably on leash. When he's not by your side, he's in his crate. What do you do when you catch him in the act? He knew at one point that he wasn't supposed to go inside, so I would make it clear that going inside is a definite no-no when you catch him in the act.. then immediately bring him outside and praise.

It's so strange that he's doing this all of the sudden. :\ My first guess would have been some kind of medical problem because it seems so unusual that a housebroken dog would slip back and start marking for no reason.