When there is a buyer and a co-buyer (cosigner), both parties have equal rights to the vehicle. This is not a situation where it's a stolen vehicle, as he has just as much right to drive it as you. You cannot have the vehicle "repossessed" and turned over to you, as it's his car too. As a cosigner, you agreed to assume full responsibility for the loan in the case of non-payment. Possession of the vehicle is irrelevant. The vehicle could be sitting in storage, and yet you'd still be responsible for the loan.
You have two options:
1. Pay the past due amount and get the loan current.
2. Allow the vehicle to be repossessed. Understand that if this happens, they will sell the car at auction, and sue you for the difference in what they sold it for and what you owe. They'll win, and in states where it's legal, they'll garnish your wages for up to 25% of your gross (take home) pay.
There are no other options.
20 years retail auto sales experience.