he probably still remembers his legion account. thats the first thing he did after nost shut down.....he listened to blizzard and bought legion and played it for the past months
of course he would have forgotten all about nostalrius
If you remember the email (as you confirm to do), and you ever restarted your account's password during playing on nostalrius, log in and search "nostalrius"
You will find a mail with your account's name inside.