Legal Question in Civil Litigation in California

You need to tell us who holds the title to the car, did the buyer know or should have known that your son has a mental illness, does that illness prevent him form knowing what he was doing in selling the car, how much is the car worth, why will the buyer not re-sell the car to you, is the buyer a person with asset at least equal to the value of the car or a business, why did your son sell the car, etc. With that information we can decide if you remain the owner as no title can pass, sale is voidable, buyer is a crook, etc.

If you hold the sole title on the car, then no one else had the right to sell it or convey title. This buyer does not 'own' the car, and can't get it registered in his name. You could file with the police a stolen car and receiving stolen property report, and see if they can 'persuade' him to return it in exchange for his payment back.