Well, he can't damage it, since you have a stake in the ownership of the vehicle. If you have reason to believe that he is damaging the car (keying it, etc) I would recommend contacting the police. The best plan is to try to agree to sell the car and split the proceeds. If the two of you don't get along, you won't want to be splitting a car. Assuming this isn't a Bentley, you won't want to get into a legal argument with him. Just come to an agreement (even if you have to give him more than what he paid for the car). Get him to agree to sign off on the sale and be done with guy.
This can be a common problem when there are cosigners. The easiest solution would be to buy him out and have him voluntarily sign over his interest in the car so you can register it in your name only. If he is not willing to do this for a reasonable amount then the two of you should agree to sell the car and split the proceeds. Your third option would be filing suit against him but as another attorney said, unless this is a valuable car this might be cost prohibitive.
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