Yeah that makes sence, (kibble) Is he eating ok, any diarrohea, lost any weight, going bold, has the owner checked to see if any lymph nodes are up in either the neck, under arm pits, back of knees,
Ferrets keep illnesses to them self until it becomes advanced,
back leg weakness is common in ferrets with beeing ill, I.e Liver, Heart, Tumour in Pancreas, Hypoglycemia,I would suggest getting him looked at at a vets that works with exotic's,
A blood test will rule out most diseases, It could just be Anemic, or low blood sugar count,