My grandfather has Alzheimer and forgot all of things about his account
He forgot all of things about his account such as apple ID password and security question Apple ID and gmail password and security question
iPhone 5s, iOS 11, null
If he can't get access to the email account and can't reset his iTunes account's password (http://iforgot.apple.com) nor security questions (If you forgot the answers to your Apple ID security questions) herself, he could try contacting iTunes Support and see if they can/will reset them for him (they will only do so if they can confirm his and that it's his account) : http://www.apple.com/support/itunes/ww