Typically, if the vehicle is registered in your name, then you are the legal owner. You should call the police and tell them that you own the vehicle and you want it back. What is his proof? Did you agree to sell the vehicle? Is he making payments for the title? Here is some information:
You may need to go to court to determine ownership if the police will not return the vehicle to you. I hope this answers your question. If not, then please reply. Thank you.