Theres actually a few different causes for this... it could be blood from a minor tear - has he been constipated at all? Or it could be from higher within his GIT... I would definately have him checked out by a Dr to be on the safe side. Keep a note of what colour the blood is too - if its bright red, or dark blackish.

When my son has nosebleeds, if he swallows a lot of blood it changes his poos to a dark black and tarry colour, which is what digested blood looks like...