St. Maarten
If you decide to go to Orient Beach - go on your own for 3 of you it will cost about 1/3 of
DCL excursion price (but you don't get the lunch tossed in). Orient Beach is beautiful and the water / waves are perfect for playing in (not for toddlers/small children).
A fun DCL excursion is the Golden Eagle Catamaran. Everyone thats done it has had a wonderful time. And when they return to port, they try their darnded to make the rest of us wish we were on that excursion. It's sailing, snorkeling, swimming and a beach party tossed into one excursion.
St Thomas
I'm one of the St. John's bigots. I would find an excursion that goes to St. John's and go snorkeling (some of the best in the world). There are good beaches on St. Thomas too. Next time I'm there I'm going up the tramway to the top of the island (if we have time after returning from St. John's)
The Ship
If this is your first cruise, I wouldn't miss the main shows in the theaters. We also enjoy the caberet shows that are between the dinner seatings (7:45 to 8:15). These are the family shows. The "adult" shows start between 10:15 and 10:30
Don't expect to see your son a lot on the ship if he get along with other kids his age. He'll probably be spending a lot of time with new friends at the stack or hanging out around the ship. It's a pretty safe environment, but set some guidlines and consequences. The guidlines are for your peace of mind, the safety of your child and the enjoyment of your fellow guests (no knocking on other doors at 3am, no crank calls in the middle of the night, etc)
If you come up with any specific questions, we're more than happy to help out with our opinions.