If your son expresses any interest in skipping grades, do it sooner, rather than later.

Strongly seconded. Hell, tell him it's an option; he won't automatically know.

As a child, I was offered the opportunity to skip a grade. I decided not to, on the basis that (my parents claimed) it would cause my socialization to suffer. It's questionable to what degree that was really my decision, but regardless...

Of course it later turned out that, in fact, I was still bored. And socialization? Well, you don't actually learn that from your peers; you learn it from those older than yourself, so skipping a grade or two would actually help. Don't make that mistake - and report back, this is still an anecdote.

Another thing you can do is hire a tutor. PhD students are usually good for this kind of cheap labor.