Diarrhea can be, but is not often a symptom of colon cancer
(constipation can equally be a syptom, but isn't usually).  More often
colon cancer symptoms include anemia, blood in stool, unexplained
weight loss, and may involve abdominal pain and tenderness or
obstruction.  Very often, it has no symptoms until pretty severe.

Most likely they gave him Flaygl (Metronadizole), for his diarrhea,
because it kills a number of common organisms that cause it.  But
it doesn't kill all organisms that can cause it.  He probably just
needs a course of a different drug.

When Gideon has an irritable bowel flare-up with secondary infection,
Flagyl will cure the diarrhea about 2/3 of the time.  When it doesn't
cure the diarrhea, they give him Albon suspension, which kills some
organisms that the Flagyl doesn't  (and is much better tasting to
the kitty).  I guess it doesn't kill the more common causes as well,
or else they would try it first instead of the Flagyl.