Besides contacting the police and law enforcement stuff-
-you have a driver's license, is he still at that address? How do you know he isn't at the same address, just with the phone turned off for nonpayment-I wouldn't count on it, but...
You might be able to backtrack him through his license. There are sometime restrictions on gaining access to Motor Vehicle records, but you might be able to do that. If he is a real scummer his Motor Vehicle address will be fake.
Depending on the area where he lives and his name you can sometimes track people down by calling possible relatives and trying to con the address out of them, calling neighbors sometimes works too.
There is also a form you can file with the post office to find out any forwarding address. Does the license have a physical address or just a PO Box?
In the end you may have to get a lawyer in the state where he lives to commence a lawsuit to gain title to the car. How about having the VIN number run? Does this guy really own the car? I could write you out a reciept for a car that I stole from an ex-girlfriend and have it shipped to you.
Does he have a record, you can sometimes find that out through the local criminal court.