Are your son’s contacts synced to any sort of cloud service (iCloud, Google, Microsoft Exchange, etc.)? If so there could be some glitch where the cloud service thinks this is your phone number and isn’t listening to your attempts to change it back on the phone.
To check, you can go to Settings -> Mail -> Accounts and look whether it says “Contacts” under any of the listed accounts. If it does, you could try tapping on any accounts that are synchronizing contacts and turn contacts off. Then change your number on the phone and see if it sticks. If it does, the problem is with the cloud service rather than the phone.
