Blood in bowl for awhile?

someone dear to me has had blood in their stool for quiet sometime now. its been more than a year that he has been having blood on and off. he doesnt tell anyone but once in awhile he doesnt flush and we find it. i just saw it again tonite and im pretty worried. he had a colonoscopy november of 2008 and everything came out good. i feel something is really wrong but i dont know what it could be. could you help me, like what kind of illnesses cause this. theres a pretty good amount of blood.