You've "talking" to the right person. Used to teach preschool and we cooked EVERY day in the classroom. I used food as a HUGE part of the cirriculum. We talked about where it grew (geography), how it grew (science), tried ALL kinds of new food in the classroom and talked about texture, taste, smell, etc. TRY TO DEVELOP ALL SENSES! Even when it comes to spices and herbs....when you use them, have the child smell it. Reminders that they are PLANTS and therefore have a tremendous amount of nutrient value.
This might be a great time to learn some more things too. If you don't know.......find out and you could do it together.
Talk about proteins, carbs (good and bad) and fats (good and bad). Discuss what foods provide each of these. Let them help plan meals. We did this in class. When they understand that balanced diets mean proteins, good carbs - fruits, veg and grains, etc...then THEY TOO can and end up LOVE helping to plan menus.
THEY CAN DO THIS!! I KNOW! I've had classes full of them. You need to take EVERY food you eat and talk about the value of it - not necessarily in detail...but ON THEIR LEVEL. Proteins are necessary to build and maintain muscle. Good carbohydrates are necessary for energy and good fats are necessary for skin, heart, etc.
Might want to find the MAGIC SCHOOLBUS book about the body (can't think of the name of it off the top of my head) but it might be a huge help in the learning process.
When you go to the store. Try NOT to go on a limited time clock. Talk about shapes, colors, comparisons, etc. When he's learned all his numbers, let him pick the cereal with the LEAST (smallest number) where the sugar content is listed, etc. Let me tell you, once you start this, it's almost like you can't stop.
They are SSSSOOO eager to learn and I've used this to "push" the reading and phonics a bit sooner. When they want to know the word "sugar" or "fiber", it's incentive to get them to learn letters and sounds of letters so they can read. Even if they know their letters, they might be able to find FIBER on the box just because they recognice the "F" in the list. I can't begin to tell you the confidence this gives thenm in making choices. You ARE doing the right thing.
I always told the kids that food choices are things that have to be made EVERY day of their life regardless of anything else so it was VERY important to learn HOW to do it and why it was important.
I have to tell you that the first kids I had when I did this are now sophomores in high school. NONE of them are overweight and all of them STILL make choices like they were taught in preschool. I know because they go to my church and I still see them and the parents. I just had a conversation w/ one of the moms about this about a month ago. It HAS made a world of difference!
Hope this helps.