Feynman's "Messenger lectures" @ Cornell begin with the example of the law of gravitation, and it's a really excellent lecture (for you, not for your son).

I think your son doesn't need answers to the questions you ask here. He wants a simpler explanation allowing him to understand the concept and relate it to things he sees in his daily life. Start with something counter-intuitive to hold his attention, e.g. with the fact that things with different mass fall with equal speed. Then talk about the Earth and the moon, tides, what an orbit is.