Want to add cell # to email account for password recovery

My Dad lost access to his email account without having setup the security questions or providing a cell # or alt email address. He called and tried to have his cell # added to his email account so he could retrieve the 6 digit code (instead of calling) in order to reset his password and get back into his email BUT... they wouldn't help him add it. He's been locked out for 4 months now. He's has had that email address for almost 20 years so there's a lot of history there that he doesn't want to lose. 

What can we do get back access to his account? I've suggested signing up for the Yahoo Email Plus but ONLY if they can help him get that address back.