Not only do you have to ask RCI, you have to have their written permission approved by Customs. It's not usually a problem getting it handled but has to be done. As Dave says, a passport is required, unlike the booking of the cruise. He'll never be allowed to board the flight without one. Get EVERYTHING in writing. Don't rely on verbal approval. Nothing verbal is binding with the cruiselines, if someone drops the ball after assuring you they'll take care of it you're out of luck.