Legal Question in Landlord & Tenant Law in California

Let's say you do and he doesn't show up and you get a judgment. How are you going to enforce that judgment? So no, if you have to hire a private investigator to find out his last name you should do that.

You can legally sue under "[First name] Doe" and amend to add the last name later, but you can't get a judgment and you can't enforce a judgment, unless you have the full legal name of the defendant.