What about the username? If he can't remember the user ID, he should know the Username because he chose that username by himself and he has to know that one. And if he can't provide the username, we can't do anything from here except he contact the support team directly.
No one is necessarily able to remember all of these details unless they choose the same alias for all their accounts, a fatal mistake many still make (in addition to reusing the same passwords). The logical solution, as user Khaled pointed out in the quote above, is to rely on the email address used to register and review old emails using the usual search functions. Without an email address, no one will be able to help him, including the support team itself.