You might try posting notices around the neighborhood to see if anyone responds. He must have been somebodys at one time or another if he's so friendly - poor kitty. Otherwise, just do what you think is best and if the owner really cares they will come looking.

Yep, I think word has gotten around that you have a very nice, kitty friendly backyard.

Hey, I hear Edwina is looking for a brother/sister. Maybe you can send him home with Sara this weekend.