I second Grand Cayman even though we've been there 4x I would still go back.. lots of places to jump in to snorkel ie Eden Rock and just walk in .. as in cemetery beach.. must rent a car and drive all around.. there is a place to swim with dolphins and to see a small turtle farm..not too exciting and he would also love the sting rays.
Another place with great snorkeling is Cozumel as has been noted.. loved it there !