he will need the raised-seal long form of his birth certificate and a notarized letter signed by both his parents giving you permission to take him, and a notarized letter giving you permission to seek medical attention for him if needed
It will depend on where you are cruising too. Bardados has just started requiring passports for EVERYONE this includes children and begins March 1.
and Aruba is expected to quickly follow.
If you are not going to a port requiring a passport then as the previous post stated official government issued birth certificate and notarized letter from parent unless both parents are traveling.