You don't say which island...

I'll start with St. Thomas:

Coral World! The SNUBA is a blast! http://www.coralworldvi.com/snuba.shtml

Mangrove Kayak/Hike/Snorkel Tour... educational and great for the entire family http://www.viecotours.com/

Atlantis Submarine & Diving: http://www.atlantisadventures.com/destinations.php?site=13

And, of course, there's always getting on a boat charter to tour and snorkel the British Virgins! There are quite a few to choose from. What kind of stuff does he like, then others can add on.

Depending on the island...

My 14 year old daughter loves horseback riding at Carolina Corral in Coral Bay, STJ.

Snorkeling never gets old.

There tend to be a lot of kids at dinner and on the beach at Maho Bay Campground and Cinnamon Beach Campground.

The beach and the swimming pool go a long way.

One thing to watch for is that the attitude towards alcohol and minors can be pretty casual. My 14 year old orders from the bar and has to make a point that she does not want anything alcoholic.

My daughters love the Caribbean and do not get bored.

Have fun, Rick