I vote for just buying certain things in bulk for cheap that he commonly eats, like the huge economy sizes of peanut butter, jelly, and cereal. And maybe start gradually watering down the juice, etc. If you do it bit by bit maybe he won't even notice!
In terms of entrees, I have learned to make double portions of certain things and freeze them, or pack them for lunch the next day, and like the other poster said, packing them up first. For example, I made a quiche the other week and I was planning on taking a piece for lunch the next day, so I put the piece for my lunch on a plate before serving and put it away to cool. I am sure that if I hadn't done that the whole quiche would have been eaten. Since that was what was available to eat, that's what everyone ate, and if anyone was still hungry they ate more salad.
Also, one idea to just stretch the budget in terms of dinners might be soups--there is actually research that people who eat soup (clear soups not cream soups) before meals actually consume fewer calories, are more satisfied, and lose more weight. Even though weight loss isn't the concern, soup is easy to make, cheap, healthy, and good! You could put some beans in there, leftover veggies, leftover meat, etc. Pretend you're a fancy restaurant and serve everyone soup and bread first!
I don't know if you/he likes cabbage, but you can take a head of green cabbage and chop it up, add a can of diced tomatoes and a chopped onion, put some water and oregano and basil and salt and pepper and whatever else you fancy, and cook it until it becomes cabbage soup!
If all else fails, hide the good stuff! :LOL