I can't activate my son's iPad I have removed my son's iPad from my iCloud account, and I have reset the iPad. I can't activate it because it says it's linked to my Apple ID and that Activation Lock is on. I can't turn this off, because when I log into my iCloud account, his iPad is no longer visible.

What can I do to remove Activation Lock and set up my son's iPad in his name?
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/.