Don't tell him he's stupid and misinformed, even though we all know that he is, just try giving him some academic papers that state a cat's instinct to hunt and kill is independent to hunger. Try this: http://www.law.ufl.edu/conservation/pdf/feralcat.pdf

I think the part about hunting being independent to hunger is on page 4 or 5, and it references another study. Highlight the relevent information and the next time he gets testy, hand the paper to him.

And this is good, from Audubon, see under Keeping Cats Indoors:

http://www.ctaudubon.org/conserv/nature/cats.htm

I don't know if you want to mention the fact that a cat will hunt less as she ages since he seems to keep the cats only to be useful. You could mention that a cat that has not received good dental care is less likely to hunt because it has a sore mouth. Cats with very sore mouths will still eat, even dry food, but they won't hunt.