Sorrento's is included in your cruise fare. You don't have to pay for the pizza. They are not open all the time however. I think they open around 11:30am and close in the early morning hours.....like 3am or so.
But pizza is also served in the main dining room at dinner, on the kids menu! It actually looked better than the pizza served at Sorrento's.
And finally, pizza is served at lunch in the Windjammer buffet.
Your son will be able to eat pizza pretty much whenever he wants!