A cup of Fiber One (the original) has 28 grams of fiber (whew!). Start with 1/2 cup, slowly work up to a cup per day because that load of fiber has it's own side effects (mainly gas) until your body gets adjusted to the higher amount. But, as we say, that too shall "pass".
Vegetables with extra fiber include those in the legume family; peas and beans. Again, sometimes with gassy results until your body gets used to the extra fiber but this will help change bowel habits.
Increase fluid intake as you increase your fiber intake.
About all you can do is explain to him that the colon typically doesn't absorb anything except liquids and since most of his has been removed (plus an important 10 inches of small intestine) hyperosmolar diarrhea from simple carbohydrate consumption is a risk factor for him.
Suggest he try avoid the sweets for one week (even a few days should see an improvement) just as an experiment to see if symptoms improve.
Add more vegetables to your diet. Fruit has limited fiber. Start adding a high fiber cereal in the morning. I'd suggest a bowl of Fiber One cereal. Start with 1/2 cup, work up to a 1 cup per day. It's the highest fiber cereal out there.