Please specify what kind of surgery, if colon cancer surgery, how much of the colon was removed and if undergoing chemo so commenters can understand the issue.

Assuming some degree of colon surgery was done since you posted in the colorectal cancer group...

Since the purpose of the colon is to remove fluid from the poop to make it semi-solid, the more of the colon is removed, the more difficulty there is with diarrhea. This is a fairly common thing.

My colon was entirely removed - one thing Mayo suggested was to take a thick, barely drinkable slurry of Metamucil to help absorb the fluid and thus tighten up the stool. Amount and frequency depends on the patient, diet and amount of functional colon that remains. Using this requires much experimentation.

Second, I found that certain things aggravate the diarrhea - sugar/high fructose corn syrup beverages and foods are troublesome. Some heavy cooking oils cause troubles, others are OK in moderation. As usual, these things will vary depending on the patient.

I would suggest asking his surgical team/post-op care team to address this and maybe refer you to a dietitian.

An internet search may be helpful as well, but try to stay with credible sources.

Hope this helps.