How to unlock a locked iPhone 6s?

So my brother recently got an iPhone 6s and he put a password in it, the next day he forgot his password and doesn't know what to do. He's not signed in on iTunes or iCloud, is there a way I can unlock it?

His password can't be retrieved. EVERYBODY know this. So is phone is now a *brick*. This makes him officially an idiot - OR - this is a stolen phone and this whole story is fake.

Call Apple and they will help you

If the iPhone is iCloud free and you just forgot the passcode for the lockscreen, you'll have to restore it via iTunes. Plug it in to a PC with iTunes and hit update and restore, if it doesn't let you because you need to access the phone, put it in recovery mode by pressing the lock and home buttons at the same time for about 15 seconds, until the connect to iTunes screen pops up and the. Proceed with the restore process.

Note, all the information will be lost and once restored you can select to restore your backup from iTunes or iCloud if you have one or continue as new. In the activation process if he did sign into iCloud it'll ask for his Apple ID email and password and the phone should have a SIM card in it to activate. If he doesn't remember the password he'll have to retrieve it from the Apple ID website. But if like you said, the phone has no such account, this won't be asked, it'll just ask you to sign in as something optional.