How can I delete my son's Apple Account on my iPhone?

I keep getting a prompt on my iphone to verify an apple account. It is the apple account of my son and I don’t know the password. If I click that I forgot the password, the email to set up a new password will be send to his email inbox that I can’t access, so that won’t help. How can I delete this account so that I don’t get that prompt all the time?