Wharton - My $.0295 worth:
He should check into the refinancing first to make sure it'll fly. Used car loan rates are generally higher than new car rates - so if he bought the car new, his new rate could be higher. But then again, with interest rates dropping lately, who knows?
But hopefully he'll find a financial institution willing to refinance the car for him. Lots of factors involved - loan value of the car, length of loan desired, amount of equity he'll have in the car, his overall credit-worthiness in the eyes of the bank, etc.
Then, if your name is on the title with your son as co-owner, have him get the car transferred to have title with his name only. You'll essentially be signing away your right to the car.
Bill