When the pancreas has areas removed , the likely result is have the lack of digestive enzymes required to digest food.
He probably should have received a prescription for CREON. Which will help replace the enzymes. WITHOUT these enzymes weight loss will result and other factors from lack of complete digestion. You need a quality GI doc and get an elastase test. The test is a simple poop test that will tell the doc how much Creon is needed. Test result below 100 = severe EPI. My friend had similar operation to remove cysts and most of the pancreas and was immediately prescribed Creon. FYI-I have EPI that came on suddenly.