The phone and all that aside, i would sugfest you put money into into your DS's PayPal account to clear it to a zero balance.
You could go down the route of trying yo get the debit written off, given his age. You probably would succeed in getting this hapoen, given the over 18 requirement to use PayPal.
However...
PayPal are very good and linking accounts and keeping records. Your son will never be able to open a PayPal account since his name and address will match an account showing (a) a minus balance and (b) closed due to fraudulent activity.
Consider your son as an actual adult, when he wants to use PayPal for normal online purchases. Your actions with regards to this payment will affect that. If PayPal matk him as fraudulent, he will struggle to open an account with them in the future.
You may also find your own PayPal accounts flagged too - given you share an address, isp and surname (?).
For this reason, I would suggest you don't cry fraud, unfair, he's too young, I'm not paying. The only fraud here is your son - he has a phone that has not been paid for.
So you pay funds into PayPal and clear the negative balance.
Then withdraw the same amount from h7s bank account to pay you back.
Then bollock him. Big time.
Online fraud is no small matter. It could have significant long term consequences if you are not careful.