mudguard:
Unfortunately this issue won't go away, it will have affected his credit. If you or someone is in a position to clear it then do it. The bottom line is the debt is in your son's name and it doesn't really matter who acknowledges what or arranges anything, it's him that will get penalised/chased for the debt.
This is the fundamental issue. The account is in his name, it's his debt. Whatever arrangements he made with the third party are not legally Ones problem.
Similarly if the third party has authority on the account and changed contact details etc it's still your sons liability. One likely haven't done anything wrong here.
It's an awful situation and it's going to cost your son money. He maybe able to pursue the third party for it but it's unfortunately probably not going to end happily for him.