There is no workaround. And don’t try to guess it, as too many guesses with erase the phone.
You can probably log into his iCloud account if you either know his password or have access to his email account. Here is how to reset the password→If you have forgotten your Apple ID password – Apple Support. If he syncs or backs up his phone to iCloud you can access most of the information on the phone. Likewise if he backs up his phone to a computer and you have access to that computer. Apple support may be able to help you get iCloud access, also; use the Get Support link at the top of this page. They can’t help you unlock the phone, however.