You are describing exocrine pancreatic insufficiency (EPI).  This is a rare
problem in ferrets.  What other signs is he showing besides the weight loss
(ravenous appetite, large amount of feces/mucoid diarrhea, anemia from
malnutrition)?
 
If it is EPI then the 2 big things to treat it would be :
1) a bland diet such as Gerber's baby food (chicken or turkey), or Hill's
a/d and 2) a powdered pancreatic enzyme product to add to his food.  Some
will also need an antibiotic for bacterial overgrowth and a vitamin-mineral
supplement.  Treatment will likely be needed for the rest of his life.
 
Hope that helps,
Jerry Murray, DVM
[Posted in FML issue 3470]