You may need to activate iMessage with the new number, Try this
1. Go to Settings
2. Go to Messages
3. Switch iMessage OFF
4. Once it's switched off, turn it
back ON again
5. Wait for it to finish activating in iMessage
6. Once it's done, turn off and restart your phone.
You may need to factory reset the iPhone to remove any other accounts on the phone, then register the phone with your sons iTunes account.
You may then need to sync with iTunes to get the carrier settings.