To whom doesn't believe that there an ipad app can be used as a telephone, try this app it's free on the App Store plus they will give you 0.99$ to use it locally and internationally.
It's an awesome app guys
I've been using MilliTalk and it's a great phone app. It makes calling anyone anywhere fast and easy. And all MilliTalk to MilliTalk calls and texts are completely free.
You can get it too in the App Store, at http://alau.me/0h7v9a.
A few more things I love about MilliTalk: it adds another line...
I think it's better to give your apple ID to your son since you didn't buy that much apps and create a new apple ID for yourself.
Try to change you email ID from that account and let him put his ID, then try to use your ID to create a new apple ID