I agree with Harry. I would have him start a BRATT diet....bananas, rice, applesauce, tea & toast. This is easy on the digestive tract and should help him out a litttle bit. Is he on any antibiotics???? Sometimes that may cause the diarrhea as well.
If it continues I would suggest getting in touch with the doctor who performed the colonoscopy. The diarrhea shouldn't persist after resuming a regular diet unless there is something else going on.
Good luck!!! ~ Goody