Give him paper and pencil, send him to the store with your budget and have him write down what he thinks you should get along with the prices.

See what he comes up with. Perhaps some things will fit with your budget and food choices and others won't. Try to explain why certain foods don't work (for $4 it's gone in one day, what do you eat tomorrow, you both need to eat and it's not something you can share). He may come away with a better understanding of what you are dealing with and you may find that you can compromise on some items on occasion.