Ok, we just did that same itinary and really like it a lot. Yes to the Sting Rays in Grand Cayman, I'm sure your son will love it. Be sure to pick up an underwater camera at home before you leave. They will be double the price there..
Good snorkeling in Cozumel and there were some other snorkeling excursions in Belize if you can't do the came tubing there, but remember this is a fairly long tender ride depending on what tender you are on, the smaller boats were much faster, but held less people..
I just posted some Voyager pics, here is the link if you want to take a look:
My Voyager pics from Feb.22 sailing