Do it! Our daughter was 6 on our first charter and we've had a blast each time. Your son will be writing class stories about it for the next couple of years. Immediate thoughts include.

Reef tour with Gumption to see the turtles
Hike the trails above BEYC
Tarpon feeding at Saba
Hike around Sandy Cay to see the crabs and wildlife.
ATV ride around JVD
Ice cream treat in West End (or anywhere)
The big hammock in Trellis
The pool at Leverick.
Of course, the Baths
Michael Beans at Leverick is a must if you time it.

I'm sure I'll think of other highlights later. Here is our video from New Years 2013/2014.

BVI Video