my son voluntarily had his car repossed, he owes more than what the car is actually worth, how much of that debt is he actually responsible for?

The answer to this question depends on what his loan documents say. More than likely, he would be responsible for the balance of the loan that is not satisfied by the sale of the vehicle. However, he should contact the company and ask them to work out a settlement with him with payments he can afford. Otherwise, the company will likely get a judgment against him. If that happens, he can still work out a payment plan to pay the judgment. It's better for him to work it out before it goes that far.