I can't activate my son's iPad
Per Apple, you will need to use your Apple ID to login on that screen, and then you will be able to remove the device from your account.

If that doesn't work, continue setting up the device using your account. Once it's set up, enable Find My iPad, make sure it shows back up in your iCloud account, then disable it again. Ensure it no longer shows up in your iCloud account. Then reset the iPad again and it should no longer have the Activation Lock in place. You can also verify if the iPad currently has Activation Lock using https://www.icloud.com/activationlock/.