Packing for kids for a cruise?

For his Christmas present this year, my teenage son was given a cruise by his grandparents. The grandparents, my son and my neice are all going on this cruise to the Caribbean. I've never been on a cruise, and I'm wondering what he should bring? His trip is next month and I'm at a loss! I'm a horrible packer and always pack way too much, so I just want to make sure he has what he needs without sending useless items. I know he'll be swimming a lot and it will be warm, so I know shorts and bathing suits. But will there be need for pants at all? And how "formal" should I go. When they go to the dining room for dinner would jeans and a nice shirt suffice, or should it be more formal?

Help!

Michelle