Unfortunately, even if he was not listed as the purchaser, if he co-signed the loan he is responsible for repayment of the loan. The other issue is if the car is repossessed your fiance may be faced with a deficiency judgment against him as well.
Your fiance can surrender the car voluntarily. He can call the lender and tell them to pick up the car, however, that is not going to help if they decide to pursue the difference they get at auction and the balance of the note (deficiency judgment).
One of the only options at this point is to call and negotiate a payment plan with the lender. I am sure you are probably thinking that is not going to solve the problem and why should he pay for a car he doesn't drive, but if he is on the note he is responsible.
If he is on the title, he could go get the car and try and sell it. If the payments continue to be missed the car will be repossessed eventually.
This is one of the major "cons" of co-signing a loan, the risk is large, if the borrower does not pay it will damage your credit and could put you at risk for a judgment.