See If you forgot your Apple ID password https://support.apple.com/en-us/HT201487
He also can use Having trouble signing in/iForgot https://iforgot.apple.com/password/verify/appleid
And he will need to see If you forgot the answers to your Apple ID security questions https://support.apple.com/en-us/HT201485
Before trying to answer his security questions is very important that he think about his questions, including his birth date, and their answers carefully and try to recall all details he can. If he don't answer them correctly enough, then he easily can lose control of his account.
Note that If he is using two-factor authentication and can’t recover his password, he usually will need to enter account recovery. See Recover your Apple ID when you can’t reset your password https://support.apple.com/en-us/HT204921.