A.S.
Try submitting to Marla Dell agency. You can find them online. They want 3 snapshots of your child. I submitted for my son, but got a polite rejection letter. you should never have to pay money to get your kid into modeling. if they ask for money, it is a scam. Agencies make their money when your child books a job. Having said that, it may be harder than it seems to get an agency to sign him on. I know there are a few other kid agencies in SF, but I don't know much about them. I am going to submit to Marla Dell for my second son. good luck!
Also, know that when you do get signed and have to go on jobs, you will literally have 24 hours notice to go to a casting call. One of the reasons that I have not pursued this very aggressively is that I would have a hard time to go to calls with only 24 hours notice. If you have a flexible schedule and can go on calls with last minute notice, then definitely give it a try. Send a catchy letter introducing your son to the agency, as well as your 3 snapshots. have fun!