Medical attention. Probably the rectal bleeding caused his blood pressure to get low and made him dizzy, hence the fall. You need to take him to his doctor as soon as possible to be checked. If the bleeding is significant or recurrs, take him to the emergency room.
News exam. Needs to be examined. Possibilities include injury to hemorroids, or lacerations in the perinatal or colon. He may have diverticulad disease or even a blledin cancer (less likely if fresh blood was present). If the patient is on anticoagulants (warfarin, pradaxa, plavix)or Aspirin containing medicines (advil, aleve, (naproxen) aspirin, or others. (blood thinners) they must be stopped until bleeding stops.
Lower GI bleed. He needs to seek immediate medical attention in the emergency room. A lower gastrointestinal bleed can be life threatening due to severe anemia. Call an ambulance right away.
Get checked. The only way to know is by direct visualization and/or other specialized testing. Although probably benign, this could be serious so it's always best to get checked unless you yourself can see the source of the bleeding as it may be an external.