Have you made sure his account is active - you can do that by going to mail.yahoo.com and trying to log in there with his full username and password. Full username is
username@domain.com. If his account isn't active, you can manually set it up at your place by going to
https://sbcreg.sbcglobal.net/DSLRegWeb/ and set it up. Then the modem should take his password and do its thing.