If you happen to catch him doing his business in the house try just simply making him sit in the corner and ignoring him for a while. Dogs seek attention whether it be negative or positive so even telling him "no" might be giving him something he wants. We had a problem with my dog going to the bathroom in the house even just after being let out (he had already been potty-trained and had been going to the bathroom fine outside for like a year) until he randomly started going inside the house. We had tried yelling "no" at him for a while, we tried putting him outside until he went to the bathroom and we tried many other things. But then I decided one time (when I caught him in the act) to just make him sit in a corner and don't let him leave for a few minutes and if he whines or anything just ignore it. I think what Benji (my dog) was doing, when he was messing in the house, was trying to get attention so when he wasn't getting it, he stopped. Maybe this will help. Good Luck!