check all the lost pet ads you can find, from the last couple of months -- newspaper classifieds, craigslist, other internet postings. Call local vets offices, shelters and animal control and ask if they have a lost dog flyer that fits his description. After a few weeks or a month people stop looking really hard. Bring him to a vet just to have him scanned to see if he has a microchip (there should be no charge for the scan; ask first). I know this is probably the last thing you need right now.
Good luck. I once found someone's pet parrot, and I have to say I made a much bigger effort to locate the rightful owner than the %$#@ owner did to try to find their bird. They pretty much gave up as soon as it flew away... I plastered the town with flyers and put classified ads in many newspapers, even as far away as Rockford (HEY I didn't know how far that parrot might have flown). The bird had some very unusual markings for a parrot, so I said they had to describe the bird to get it back. Soooo everyone who had ever lost a bird within 100 miles of me was calling that week; the phone was ringing off the hook. Eventually the real owner called and the bird went home. After that I stopped answering the phone and put a message on the answering machine that said, "If you're calling about the bird we found, it has been reunited with its rightful owner." fun times.
