MIME sections within the text body of an e-mail have no means to set a
read-only file attribute - because it isn't a file yet (i.e., you have
to save the MIME part denoting an "attachment" before it becomes a file
and can then have the read-only attribute set on it). So your boss is
saving it somewhere that makes it read-only. Your boss will have to fix
that problem, or get the IT to do his work for him.
Of course, maybe what you really meant to ask was a Word or Excel
question and NOT an Outlook (e-mail) question, like you configured Word
to set the document to read-only mode so that's what your boss got when
he opened it (i.e., the read-only action is a Word thing, not a file
system thing). Guess you'll have to wander through Word's options
looking for whatever allow you to set your documents to read-only (which
only applies within Word itself when viewing that document). In that
case, you'll probably do better asking in a Word group on where is the
option that you haven't found yet that configures your .doc files to be
read-only when someone else views them in Word. There are newsgroups
for Word.