The pancrease still can have enough functionality to break down foods; however, being that he's homozygous delta f508, he probably still needs enzymes. Breast milk can easily be broken down, in fact that's how some parents discovered their children had cf when they switched from BM to formula or whole milk.
DS never had greasy, bulky or oily stools as a baby and had constipation issues, along with really stinky stools in technicolor. Since his digestive issues including an obstruction caused by adhesions last summer he's actually had normal, brown stools. We've upped his enzymes, no more greenish flecks.