When first setting up a phone and you enter your Apple Account and Password, it turns on Find My so in case you lose your device or it is stolen, you can track it and the thief will have not way to reset it and set it up as theirs. The iPhone will automatically be disabled when the Passcode has been entered incorrectly to many times.
Now since this is your phone, you have 2 choices.
- Gain access to your account by following the steps here to reset your Password. After you are able to gain access to your account you will be able to Factory Reset the device and your son can set it up with their Apple Account.
If you forgot your Apple Account password - Apple Support
- Remove the Activation Lock by submitting the original purchase receipt from Apple or Authorized Apple Seller. After this lock has been removed, you son get set the device up as theirs.
https://al-support.apple.com/#/additional-support