Way back when I used to breed mice.  One of the first litters that I bred
had a tiny female mouse in it, she never got any bigger than a baby mouse,
although she was perfect and healthy in every other detail.  She was also
sterile in that she never bred (I am not sure if this was true sterility or
a physical impossibility for a male to mate with her), and she lived to die
at a ripe old age (for a mouse).  Interestingly enough she was also one of
the "alpha" mice of the colony.
 
Not sure if your little boy is exhibiting a similar genetic freak or
whether this is a recognised genetic pattern in ferrets.  I did read
somewhere that people used to breed for "greyhound" ferrets but not sure
if this is still going on either.  If he is perfect in every other way it
would be interesting to see if he would breed true to form, personally I
love teeny ferrets :)
 
TK
[Posted in FML issue 2922]