Is his name on the title? If not, you will need to send him a letter stating your intentions to reposses the vehicle unless he starts paying for the car...
You will need to do some investigative research to figure out where he lives, works, etc. You can do an internet search for him or hire a P.I. to do this for you...
If his name IS on the title, there is nothing you can do at this time but go after him for his portion of the vehicle in small claims court...
Either way, you will need to find him...