To deduct a bad debt arising from nonbusiness activities, such as a loan to a family member, you must be able to show (a) that you actively attempted to collect on the debt, and (b) that the entire remaining balance of the debt is uncollectable. You must also take the deduction in the year the debt became uncollectable. You would do this on a Schedule D with your itemized tax return.
For this situation, though, you have a problem - if you just sit on your hands, you're not taking reasonable steps to collect the debt. The debt is secured by the vehicle. If the son ratified the debt, it's still due and payable. If not, you can repossess the vehicle and sell it to recoup your losses, consistent with your state's auto repossession laws.

