Dear mister, unfortunately the first thing to think about are complications of previous diseases and surgical procedures. It is common to experience digestion disorders after colon and liver cancer
especially if treated surgically or radioth/chemoth. I understand your father knowing everything he went through, but you should definitely convince him to do some diagnostic test( Ct scan
and gastroscopy) to rule out recidives of tumors but also any new abdominal pathology which can be treatable. If there is mechanical obstruction or colon mucosal damage, diet wont be helpful. You can only care that he doesnt lose to much weight or become dehydrated.
Wish you good health, Dr. Ivan Romic (Rominho)