I agree with the above response to your posting. Rectal bleeding is never normal and should always be medically investigated until a definite cause is found. Normally a colonoscopy is a comprehensive test to discover the source of bleeding if the cause cannot be identified by visual examination (e.g. haemorrhoids, anal fissure etc). Google rectal bleeding and click onto one of the many sites - the Medicinenet site is particularly informative. This should inform you better for your next consultation.
I suggest that your next appointment should be with a GI specialist and, if necessary ask for a repeat colonoscopy.
regards
Morecambe
Not sure where you live but here in N.Y. we have a Disgestive Disease Center. I would take him to another Gastroenterologist because thats a huge red flag for colon cancer. I also don't know how old he is but my friend just lost her son to colon cancer at age 21. By the time they found out he had it, he was in stage 4. Reason being, they do not check anyone younger than 40 sometimes 50 for that disease. Take him right away and good luck.