Hi I had the whipple at 60 years old (had cancer). As for the oily stools, it's because his body is not breaking down the fats. Get the Dr tpo prescribe some enzymes. You take them each time you eat. How many depends on what you eat. I had the same problem and the enzymes do the trick.
Also, coconut/macaroon cookies help with the diarreah.
God bless