Tell him to see If you forgot your Apple ID https://support.apple.com/en-us/HT201354
He also can use Having trouble signing in/iForgot https://iforgot.apple.com/password/verify/appleid
And he almost certainly will need to see If you forgot the answers to your Apple ID security questions https://support.apple.com/en-us/HT201485
Let him know that before trying to answer his security questions it is very important for him to think about the questions and their answers carefully and try to recall all details he can. If he doesn't answer them correctly enough, then he easily can lose control of his account.