Betty,
You did not mention the age of your ferret, but my first suspicion would be
adrenal disease.  The balding pattern - starting at the base of the tail and
progressing upwards is a good indicator -- though it has also started at the
shoulders and worked down to the tail in past experiences as well.  I would
search for a vet in your area that is familiar with adrenal symptoms and
treatment.  Chances are if the affected adrenal gland is removed he will be
just fine and regain his coat.  If left unattended too long the gland could
eventually effect other organs.  So check this out!
 
Fondly in ferrets,
Alicia
a shelter Mom
[Posted in FML issue 1912]