I'm not a vet or a ferret expert, but mounting behavior in a fixed male can
be a sign of adrenal disease.  1.5 years ago, my baby Gunther, who never
showed any interest in such things before, starting aggressively chewing on
our female's ears and repeatedly rubbing his belly on anything round he
could find.  He had one adrenal gland removed last year, but unfortunately
it wasn't enough.  He's got an appointment for August to remove all or part
of the other one.
 
See the medical FAQ for more info on adrenal disease.  Other signs of the
disease are thinning fur starting on the tail and moving up the body, loss
of weight, and a hyper, somewhat aggressive attitude.
 
Good luck.
 
Norris (for Gunther, Nico, and Ozzie)
[Posted in FML issue 1997]