Agree re Craigslist -- when I lived in So Cal, I was able to help dogs who lacked tags/chips find their way home because their owners placed a CL ad! It works!!!
Also, look for Facebook groups called "Lost Pets of..." (Los Angeles, Antelope Valley, Simi, Ventura, etc.), and get him up on all of them in Southern California. Don't assume he stayed local -- I know people there who have found dogs on the side of a highway in Southern California while going somewhere, and picked them up to get them safe, then tried to find the owner from far away. It's such a mobile place, he could be safe in the home of someone pretty far away who's trying to find you, but doesn't know your area and won't see your signs.
When you are checking shelters, in the greater Los Angeles area, be aware well-meaning people also take dogs to shelters far away from where found under the view that the far-away shelter has a lower kill rate than the local one. I've heard of people driving dogs from Antelope Valley to West L.A. (or Pasadena etc.) because it's perceived as a "better" shelter -- they'll sometimes lie about where the dog was found to get the shelter to take the dog. These people don't realize how hard they make it for the owner looking for the dog to ever find it again, unfortunately.
Be sure to send out your picture flyer to Burbank, Westside and OC GSRs.