Did you have the vet thoroughly check his teeth? Does he have a chipped
tooth, you know, the 4 long fang looking teeth in the front? Does he have
a loose tooth or darkened teeth of any sort in his mouth? Sometimes you
have to look very closely and carefully to see the chips and breaks in
them. I have had ferrets in the past with teeth problems and they can be
fed soft food but cannot "crunch" on their own till the tooth problem is
fixed.
I learned the hard way after trying everything but the teeth, even my vet
missed it. I just thought about it one day and realized maybe a sore tooth
or a mouth sore would make the ferret not want to eat normally. Since I
saw no mention of teeth in your post that is what I thought of right away!
Good Luck,
Joey
[Posted in FML issue 3017]