Sometimes the prescribed Meds can have side effects it may be worth taking him to the Doctor to see if the meds need adjusting or changing, I would do that first .When you say you have a structured enviroment does that mean you have rules etc that he is not used to ,how does he take them is he happy or does he argue back,does he seem to like being with you,happy when he arrives, something is upsetting him at your house, have you asked his other Family if he urinates with them. Get his Dad to play games and outside sports with him, he may respond more and tell you why he is doing it, children nearly all say they dont know.Ask the other Family for input.