I would simply confront him about it. Be point blank. Tell him he's making a mess in the bathroom when he uses the bathroom, and here's the cleaning supplies to clean it up with. If he refuses, then I'd point him to the nearest hotel, and tell him if he's going to be gross in MY house, he's not welcome.
Normally, I'm in favor of letting the blood relative to the offender handle it, but in this case, I'd take care of it myself. That goes beyond the realm of common decency.