There are many places where a device will ask for passwords, but if you are seeing the one mentioned in activation lock then you are facing one for which there are few options. You can enter the user email and password (and one does exist because it could not have been set up like this without your brother providing one). You can go to an Apple Store with the original receipt so your brother can prove it is his. You can recycle the phone and buy another. That's it. As I said earlier, this is a theft deterrent measure and Apple is very strict about it because before they introduced this feature it was too easy for a stolen phone to be reset and sold.
People need to learn that sometimes passwords are like bank accounts. There's a lot of financial investment stored behind that key and they need to keep their password as available and safe as a car title or key to a house.
Does your brother not have any email address?? That would likely be his Apple ID. Then you need the password:
If you forgot your Apple ID password - Apple Support - https://support.apple.com/HT201487