Any sign of a collar or anything? Have you looked around for "lost cat" signs? If he's a teenager, he's old enough to be neutered, by the way. Call your local paper, they often list "lost" or found" pets for free. Have you contacted Animal Control to see if anyone is looking for him?

If no one is looking for him, maybe you can put a notice in the paper or on craigslist or even Freecycle and "sell" him for ten dollars - something cheap enough, but not free, so he won't fall into the hands of bad guys ... Freecycle doesn't allow you to charge anything, so you'd just have to careful about who you let take him.

Does your vet have anyone who might foster him?