If his name is not on the title, and it is soley in your name, you can go to the police department and tell them that he will not give it back, and show them the title.
First, call him and tell him you want your car back. If he refuses. I would make him aware that he no longer has my permission to drive it. The police would be notified.
These is simple, if he's not on the title then call the police. If he refuses to give your car back this is now in theory stealing. The Title is the evidence of whom owns the vehicle.
I guess you could easily go to your boyfriend to ask him nicely to give you back the car and if he refuse you may go to police and tell that he stole your car. It may work to scary him.
If he isn't on the title and is refusing to return the vehicle, there's probably a criminal charge and a day in court awaiting him. I would call the police and file a report.
