Hello, this is Dr. David. I have read your question and I'm ready to help.
just after a colonoscopy, his bowels are most likely taking time to get back to normal. so it can be normal to have thin and flat and loose stools since he just cleared out his colon for the colonoscopy. and the colonosocpy would have seen any signs of colon cancer. so colon cancer would not be causing his stool changes.
sometimes if you get a temporary viral
infection of the bowels or some mild food poisoning, you can have thin or flat or loose stools for a few days.
if this happens, soemtimes it helps to eat more fiber in your diet or to eat more constipating foods like rice, cheese, potatoes which can help slow down the bowels and help cause more formed stools.
also sometimes adding more yogurt into his diet and probiotics daily or multiple times a day can help put good bacteria into his bowels and this will help his regularity as well.
does he eat yogurt daily?