Karen,
 
I'm no vet, but I still believe your ferret has adrenal symptoms and should
probably be opened up.  (BTW, is he Marshall Farms -- two blue dots on right
ear?)
 
I do believe most hair loss occurs from bottom end up, but each animal is
individual and I would take that into consideration.  Plus, you did mention
that the hair loss is bilateral, and with adrenal problems, usually is.  The
lethargy you mentioned is also a symptom of adrenals (as well as many other
diseases), but the one key thing you mentioned was he was getting "heavy".
Do you mean he's actually getting heavier, or he's beginning to develop a
pear shape when you hold him up?  The pear shape (as well as loose skin
around the back legs) is another symptom.
 
I only know cause I've been through it, and helping out at a shelter have
seen it many times.
 
Good luck!
 
Amy & Dave (and their gaggle of giggling ferrets)
RIP Ian and Elektra
[Posted in FML issue 1627]