I'm not sure what you mean about an explanation for "how gravity works". There are lots of different explanations for this in different theories of physics, but even a profoundly gifted five-year-old would have trouble understanding most of them (one interpretation of quantum physics actually explains gravity as an effect of TIME "pulling" on objects)!
That said, I understood the classic "rubber sheet analogy" of Einsteinian gravity when I was not much older than your son.
If what you're looking for is more of an observational description of gravity - ie small things fall toward big things, things speed up as they fall, etc etc - I'm sure you can find plenty of videos. Try Discovery Education; their library is pretty extensive (I've used up my trial membership, sadly, so can't run a search for you).
