Yes, NCLS policy- they won't let you into an age group- if you aren't that age as the first day of sailing. its a stict policy that most ships follow- expecially the newer ships. Your son will be fine. there are PLENTY OF kids that are just turning a year older...within a matter of weeks even. he will be just fine. he will be able to meet a whole new bunch of friends too. He will be sad at first, but he'll get over it. And the 2-5s and 6-9s are in the same room. The 10-12s are in the teen disco during program hours. All he will have to do is just look across the room and see his friend. not to fret, he will be just fine.
-I'm a YC for NCL.