One of my favorite authors is Henry David Thoreau, and this is a quote from him: "It has been written that we are no greater than our dreams." We all generally accept teaching a child the importance of having dreams, but a child must also learn that all dreams require some kind of knowledge to make them come true.
When I was five, I read the story of Icarus and decided to make some wings without glue so that I could fly and not have to worry about getting too close to the sun. I made my chicken feather wings, climbed to the tallest peak on our roof, flapped my wings, and jumped. Needless to say, the impact with the ground was very painful, but even then I still believed that if my wings had been constructed right, I could have blown. When I was a teenager, I saw someone hand-glide off Sandi Peak and soar like a beautiful bird.
The dream was the same as my chicken feather wings: the difference, these wings were built with knowledge, the kind of knowledge you get from math and science classes or books.
If your child says that one day he will build an airplane that will protect passengers if it crashes, do not say, "Well, you never will be an engineer with the kinds of grades you have in math." Do not kill the dream. Instead say something like, "That would be great, and if you really want that, I'm sure you'll learn the things you need to know to be an engineer." He may just become a math whiz if that really is a dream he is serious about.
The concept of knowledge being one of the keys to making dreams come true is one of the best motivating concepts for learning. Stories about people who used knowledge to make their dreams come true are very good tools for getting this concept over to a child. The story of Thomas Edison is a good one to start with because his story also demonstrates that success in school is not nearly as important as individual learning.
If your child is learning for dreams, then learning becomes a way of life, not just something he is forced to do in school.