Do you have the $$$ to take it to the vet? It should be given a good exam, treated for any parasites and infections, then scheduled for a spay or neuter. If this is not within your budget I would call some of the shelters in your area and see if one of them would help this cat. Putting posters up in your neighborhood and a placing a found ad in the paper may also help to find the cats owners.
:Wavey: Welcome to TCS! You are a real hero tonight, caring for the kittycat. If it's possible to make a run to the grocery store, you could provide some kibble and also a dish of fresh water. Thirst is often a big problem unless your area has fresh streams nearby. Most gutter water is contaiminated. By the way, don't give too much milk because many cats are lactose intolerant. And the cat may enjoy a cardboard box with a door cut out for a shelter. You could check the lost-pets ads and place a "found pet" ad in both Petfinders and in the newspapers. Vet's offices are also another place to post ads. I'm sure that more suggestions are on their way!
Meanwhile, please accept my thanks & appreciation for your efforts in helping this cat. People like you ought to be cloned! (just kidding, I don't believe in cloning but you get my drift)
I too want to thank you for your help for this little kitty. You are one in a million to help him. If he knows that you have helped I'm sure he'll pop back at some time. Don't worry too much in the meantime though, you've done what you can for him. No kill shelter is a great idea. Alternatively you could take him to the nearest vet. Make sure you protect yourself though before you pick him up, he may be very scared and is likely to scratch.. just cover your arms and hands if you can. You are a lovely person to be so concerned though..