Contact the company that holds the loan and ask what you can do. It might require that your son qualify for a loan on his own, or, that you turn the car back over to the dealer/lender. I am sure they can answer your question. And the answer might be that you're stuck.
As a cosigner on the loan, you are probably not liable for damages inflicted by the driver or the registered owner. Assuming YOU are not listed as the registered owner of the vehicle, you should be okay from any judgment so long as you are not directly involved in whatever incident might occur.
- Carl