If reduction of smell is not him having low grade dehydration (which,
BTW, is why some meds decrease sense of smell) then you need to look for
other possible causes.  For example: does he have any signs of allergies?
 
In humans the sense of smell decreases with age.  we have NOT had ferrets
have any evidence os their senses of smell or of their senses of hearing
decrease with age in our family over 23 years, but we have seen ferret
sight decrease over time.
 
Warming food increases the scent and is advised for ferrets with reduced
senses of smell.
[Posted in FML issue 4686]