Forgot Apple ID password and birthday

I have been trying to reset my father's phone because he completely forgot his apple id password and the birthdate on his account is wrong. I'm lost and I have no idea of what to do because the account is even locked on icloud, and the email it sends the recovery email to is the icloud email that is locked? And the other option to enter the birthday for security questions is wrong as well. I don't think there's another way to fix this, but I'm not sure. We tried resetting it but it requires the activation lock and we face this problem again. Please help!