Legal Question in Civil Litigation in California

There is no liability for not working under an employment contract, and in fact if you and he had any kind of agreement by which you worked for him and he agreed to compensate you for the work, you had an employment contract anyway. Your remedy for him not paying you is to file a claim with the Department of Industrial Relations, Bureau of Labor Standards Enforcement. Google that and you will find their website immediately. I believe it has a tab for filing a claim online. Since yours is a little bit of a unique situation (their usual claim is just for things like missed breaks or unpaid overtime) you may want to look up your local office and go in.

By the way, the State of California Division of Labor Standards Enforcement (Labor Commissioner) does not care AT ALL if you were working off the books. They will go after the employer, but they will help you just the same as they would any employee. Don't be shy about paying them a visit.