From your other postings I see that you have found Kevin's birth info on the internet.Have you obtained any of his birth cert info? Finding full info on his mother, might give you a few clues about how to find him.
www.vitalrec.com gives you info on how to order birth,marriage,death certs.Read the instructions and you will see that you might be able to get his birth cert if you show relationship and "reason" for obtaining a copy...I would think that "medical family history" might be a good one or "need to notify medical family history" if that might fit into your circumstances.
Also, what are the chances that he started using his mother's last name, or that mother remarried and he was adopted or took on his stepfather's last name?There are a number of Kevin W. Johnson's in CA.One listed in San Francisco, which is nearby.www.switchboard.com is a free internet white page telephone/address directory.
www.locateme.com has been helpful to me, but there is a search fee.You can go there and read about it first.They can run people with a first name and birthdate and have some kind of database that includes voter registration and perhaps some DMV type records.Their response, when you give credit card # is within a few hours.I found them helpful the one time I tried it.