In addition to standard packing (clothes, swimwear, diapers/pull ups, toiletries,) I'd recommend taking some prepackaged snacks like goldfish or gummies just so you know you have what he likes easily accessible. Generic meds like children's ibuprofen. Sunscreen for you all and hats. Any loveys or comfort objects. A costume for pirate night. Glow jewelry for pirate night. Some toys for the state room (stickers, cheap Legos that you don't mind losing, that kind of thing.) A cup with a straw lid or flip top bottle to keep full of water or juice.
Remember that only potty trained kids are allowed in the pools. No swim diapers allowed.