Don't feed him anything in the evening, no treats, no late snacks. Walk and walk him, praise him endlessly when he "goes" outside and get him really used to the idea that outside is the only potty area. Clean the spot he usually does it with special stuff from the pet store that eliminates all trace of the odor. And you might consider crating, as mentioned, or not allowing him in your room at night for a time. Can he get in and out of the room, or is the door closed? How close to bedtime does he get to go out, and how early do you and he get up and out?