My name is***** am sorry to hear that you are having this issue.
I will be glad to assist you with this.
You can de register the Kindle and then register it with his email.
Follow these steps please.
From your device:
Then you can register again.
Please reply and let me know if you need any other assistance,
Also, please take a moment to rate my service by choosing one of the stars at the top then click submit.
Did that work for you?
Please let me know how if you need more information.