As long as there are other children in his age group, he will probably have a great time. Princess Cays is the best especially with a special area made like a Pirate Ship just for children including the attention of the Children's program staff. If you are stopping at Jamaica be sure to take in Dunn's River Falls and climb it with him. Also, go to Swim with the Sting Rays in Grand Cayman, he will love it and the water is only up to your waist (on a sand bar) and he will have a life vest on. This is a must especially if you have never done it.
As far as soda, the buffet and dining room can provide regular fruit juices and in the buffet area you may also find lemonade or fruit punch available all day. Celebrity & Royal Caribbean offer a ''soda card", but I don't believe Pricness has adopted that program yet. For reasons of cleanliness, you probably won't find refreshments available in the Children's room but he may love the independence of being on vacation and not having to stay with Mom & Dad every minute.
Also, I would recommend early dining for your family (6:30 p.m.). Kids finishing dinner at 10:30 p.m. is not usually the best bet! Breakfast & Lunch on Princess are usually open seating (you go when you are ready 7:30 a.m. - 9:30 a.m.) etc. in the main dining room and of course the buffet is open continuously now offering 24 hr. dining even for an alternative evening dining venue.