I would definitely ask around to neighbors, they might either know who the dog belongs to or be able to tell you if he's been hanging around the area a while. If you don't have any luck with that, I'd take him to your vet to be scanned for a chip and then call in a found dog report to AC. When I found a dog last year I emailed them a picture. I would also check craigslist and post flyers. If you think he'll be staying with you for more than a day or two while you look for an owner, I'd also pick up a capstar for fleas and a dewormer from the vet. You probably should keep him separated from your dogs, but I found that to be easier said than done, and after I got the stray I found de-flead I ended up letting my dogs around her.