Legal Question in Real Estate Law in California

Who is the registered owner?

I don't see that you have any rights at all, either to take the car or to resell it, without your son's full cooperation.

If and when he fails to make a payment, you might have a right to sue him for breaching his agreement with you, probably an oral contract, to be primarily responsible, but even then, judges are inclined to think parental financial assistance of this sort is probably a gift rather than a business transaction.

The lesson here is that becoming a co-borrower or guarantor is a risk for which there is often no reward.