Travelling with a four year old can be tricky. They like to snack on things that may not be healthy and kids at that age put things in their mouths that they should not do.
I am sorry that your son got sick but he probably was already feeling when he got on the ship.
Carnival Liberty New Year's Eve 2007
Liberty of the Seas 5/2008+11/2009
Allure 1/16/ 2011
Independence of the Seas 12/29/2013 Top-notch!