Maybe you could take the approach in a general way since normal arguments
don't seem to be working?  I mean his grandfather probably only took a bath
once a week, maybe didn't have a phone, no new medical procedures, modern
antiobiotics, dentures etc.  etc.  Does he still live that way or what?
(I'll bet he buys canned food at the grocery store and doesn't can
everything himself).  Granted everything new is not wonderful but maybe its
time he gave something new a try.  (I wonder why he even has ferrets if he's
not really handling them or trying to figure out how to best take care of
them.) Also, in case your trying use simple reasoning with him (doesn't
sound like it will work) hasn't he noticed (this is not so scientific just a
simplistic observation) Dogs and cats that are meat eaters have canines that
are seperated in their mouth, and - so do ferrets.  Whereas rabbits which
are vegetable eating animals have the two teeth in front next to each other -
so doesn't it seem more likely that ferrets should eat a meat based food?
You could also tell him that cleaning up will be a whole lot easier than the
runny poop that milk and bread must produce.  Maybe send a bag of food home
with him.
 
Good Luck!!
Miss Mary
[Posted in FML issue 1654]