I've heard California Natural is good for sensitive dogs.

Maybe you can start putting up posters now? Try to get the word out to as much people as you can. Call the shelters and tell them that you found the pup and if anyone is looking for a dog that fits that description, to contact you (also call the shelters that aren't in your area. If the pup has been loose for a while, it may have come from a home a lot further from you.)