Dog looks at a great weight to me, I agree with the others. That was nice of you for taking him in. Someone's got to be missing him. I know you said you tried Facebook but you could also post a found ad on Craigslist. ALOT of folks scan Craigslist ads you'd be suprised. I'd like to think that if the original owner found you that after proving ownership he'd at least reimburse your vet fees.
Oh I didn't see you mention it, have you dewormed him? Wouldn't hurt.
I had the ad up for about a week and kept getting people that claimed he was theirs but couldnt describe anything about him other than what you could see in the picture.
I havent wormed him but the vet swabbed the q-tip and studied it for a few minutes under the microscope, I am willing to worm him though, I know there are many methods but what seems to work with these dogs?