Please explain why does the doctor not let my dad drink water after his colon cancer removal surgery?

Ileus: After abdominal surgery, especially for colon, the combination of handling the intestines, and surgery itself leads to "bowels going to sleep" which means that they do not move in a coordinated fashion. It takes several days for the bowels to "wake up" and if you give patients food while the bowels are asleep they can become distended and vomit. Usually we wait until the patient passes gas.