Maybe nothing.  If there are no stools at all, though, he may have
an intestinal blockage.

According to Dr. Bruce Williams, DVM:

    Ferrets occasionally have dietary "indiscretions" and may get hold
    of something that is not particularly to the GI tract's liking.
    They may get loose or discolored stools, and if no groceries are
    going in for a day or so, their stools will lose volume and may
    become somewhat thin.  As long as they are playing and acting
    okay, they can usually tolerate this for 48-72 hours.  If it goes
    on any longer than this, or their play/sleep cycles become
    affected, then it's off to the vet for a check.  Most problems
    resolve within 72 hours on their own.  If it doesn't, then there
    may be a problem.  (However watch for dark tarry stools - they are
    more than just discolored and indicate GI bleeding.  If you ever
    see these - go see your vet.  Likewise for profuse green
    diarrhea.)  A one- to two-week course of Laxatone is also a good
    idea following changes in stools.  If there is some foreign matter
    in the intestine, it will help it move along, and, at any rate, it
    won't hurt.

    I caution everyone - don't throw out those abnormal stools without
    going through them (pick them apart in a bowl of water) and seeing
    if there is any foreign material in them.  It may sound "gross",
    but it can tell you if your ferret is eating something it
    shouldn't.

Dr. Charles Weiss, DVM, adds that GI parasites such as giardia and
coccidia can sometimes be the cause, though it's not common; and even
lymphosarcoma may cause funny-looking stools.  Both of those will
generally present other symptoms, too, though.