I would agree with Tom. Middle of the ship, lower is always smoother, the higher you go the more you'll rock. However I would do an outside cabin, inside can be very dark and not to mention closed in! When he gets sick tell him to look beyond the horizon that should help a bit. We sail quite a bit and it will help to ease his stomach. Plus keep some crackers and soda water in your cabin at all times.
Enjoy your cruise