Well I seem to have found a dog....?

First have it scanned for a chip. Just because it's in poor condition right now doesn't mean it was abandoned, possibly it ran away (since it does have a collar). Place ads on craigslist and in your local papers, put flyers in vet offices & call your local shelters to report you've found him. If his owner doesn't come forward in a decent amount of time, then you can make up your mind whether you can, or want to keep him. You can also start calling local no-kill shelters and rescues and see if any do, or will have room for him