In addition to Bob Church's chicken gravy (which I highly recommend and
which you can modify for insulinomics by omitting the Nutrical and honey,
FYI), Hill's Science Diet A/D food (which is only available through a
vet, as it is a prescription food, but is pretty much available through
most vets) is a great food for trying to bulk up and get nutrition into
ferrets, cats, dogs... It's also gentle on the tummy.  (You can try
warming it in the microwave to help make it even more appealing to
them, too, but I daresay the smell of warm A/D won't be too appealing
to YOU! ;) )
 
Nutrical, if your guy is not an insulinomic, is another great
high-calorie, nutrient-rich supplement available at your vet's office,
and is generally well-received by the ferret set-it has corn syrup and
smells kind of like molasses (not a good supplement for an insulinomic in
general once you've gotten their blood sugar regulated, and should only
be used VERY short-term as an aid in getting an insulinomic's blood sugar
up-overuse in this capacity can cause worse problems).
 
A vet tech and ferret owner, not a vet,
 
April AC
[Posted in FML issue 4482]