Unfixed dog just started marking in the house (+some extra drama)?

BYB is right. Tell them get him fixed or they have to take him back. I don't see how they can want their dog, but not even visit him. what ignorant people. You obviously would provide a good safe home for him. People sometimes I swear. Well try cleaning with special pet cleaners and everytime you see him try and pee make a loud noise and put him in a 'time out' area. Good luck.