iPhone can be used as long as it functions, but
eventually Apple will disable it and it will no longer work.
Depending on iPhone's history, iPhone may be considered
stolen with attendant legal implications. Should someone
in possession of the iPhone become a person of interest to
local authorities and they search on the iPhone serial number,
the situation may become more than it was to start.
You cannot enable the serial number. Apple considers the
iPhone to be non-existent as a functional device.
A further reason for the error message may be that the iPhone
was stolen from an Apple store in a smash-and-grab or other
robbery.
Hopefully, your brother did not pay much to the unknown person.
Consider it a life lesson and purchase the next iPhone from Apple
or an Apple authorized iPhone retailer.