Legal Question in Real Estate Law in California

As a registered owner he has an owership interest in the car so can not steal it. Under some circumstances you might be able to file a civil suit for him depriving you of useage of the car but this is a civil and not criminal matter. You could get into trouble if you report it stolen.

No. You can call, but be prepared to get in trouble for making a false police report.

You can't steal what you own. Mr. Roach is right, you can do what you want but it won't go well.

I agree. Further, theft requires an intent "permanently to deprive" the owner. Even if the taking of the car were a crime, which it is not, it would be some lesser offense such as joyriding, unless you also can show that he has no intention ever to return it to joint possession and use.