Help your child learn where food comes from by planting a garden. Even a little patch of dirt outside can turn into his "farm." If it's winter or you live in a city, make an indoor garden on a counter near the window. Plant radishes, bush beans, and lettuce: They all grow quickly, so your child will be able to see the fruits of his labor in short order. Help your child plant, water, and harvest his crops. And be sure to celebrate when you're done! If you can, take your child to visit a real farm to watch the planting or the harvest.
All of this teaches your child about consistent care and gives him responsibility, both of which are more important than the actual crops themselves. But don't expect too much from him at this age -- he'll need all the help from you he can get!