It's a lovely cruise and you will have much to do or as little to do as you like. Your son will still need a sports jacket and perhaps a suit for a few of the evenings if you want to eat in the main dining room. It is not all casual. If he is used to fine dining in NYC, then a coat is the norm for the better restaurants. The Navigator vibration is mostly in the Show Lounge and in the open seas. On this cruise there was almost no vibration that we felt. Book midship and you will be just fine. We have been on the Navigator twice and had a wonderful time on each cruise.