My daughter is going through the same thing. She has a little teeny terrior that showed up on her porch and wouldn't leave. She tried to find the owner with ads in the paper, posters in the neighborhood, going door to door. Then the little girl got Parvo. She had her treated for that and finally bought little bowls and a leash and made her part of the family. She was the perfect little puppy for their little family.
She has been missing now for about two weeks. They have done everything you are doing and I don't think there is any more you can do. She does keep checking the pound and humane society regularly in case they overlook her inquiry.
I hope you find your little guy. Don't give up.