The problem here @Lala65 is if the airtime part of the contract is transferred to you your Son would no longer be liable for any repayments and any time remaining on the contract and repayments will be in your name.
If your Son is on an Evo contract, he would be have to continue with the device payments. If the device part of the contract is paid off, he will be released from the airtime part of the contract. There is further information here: Evo
Presumably, the discount you are being offered for an additional line, would be for airtime only and not the phone. A further consideration would be you would be hard credit checked and the new device loan would be on your credit file.
