Did the owners come looking for him the night he spent with you?

If not, I say poof him. We did that with Grady, 7 years ago. He was 6 months old - not allowed in the house because one of the kids was supposedly allergic. They fed the cats by throwing dry cat food on the garage floor. They lived two doors up the street from us - had 3-4 cats, not one of them neutered or spayed.

We kept Grady. Not once did anyone come looking for him; not once.