Make sure your son is comfortable in the water. Lots of hats, sunshirts and sunscreen to avoid sunburns.
We took our son when he was 8. Took him to a dive shop here in Seattle and they had an instructor teach him how to snorkel in the pool. Best idea we ever had. He was confident and safe. He made a flotilla of coconut ships and had an epic battle on Camanoe Island, he danced and hung out with the band at Bath and Turtle. He fished off the back of the boat. You can buy a cheap fishing rod and reel at the chandlery behind Village Cay.
We bought him a gameboy and he had two hours a day he could play it. He had a Pirates Journal that we wrote in every night. He still has his pirate hat and eye patch (he is 33). Teach him how the dinghy works. We spent one day at the pool at Bitter End. There were some local kids there and he had a great time.