ask them....................

Demand the money from the boy's parents.  You need to pay the bank back very soon, or they will close your account, sue you, possibly have the police charge you with fraud, and a lot more.  You will regret it if you don't pay your debt.  Don't lend your phone again.  If his parents don't pay you, tell them that they are leaving you no other choice than to sue them and your nephew, and do it.

For the third time, you need to TALK TO THE BANK. Yes, you are liable, whether or not they will work out a repayment plan is up to them. Nobody on here can tell you what will happen. Call your bank.