Cruises are typically great for kids, especially teenagers, with most of the mass market cruiselines having activities and areas geared specifically towards kids and teens. But for most kids a transatlantic sailing will be quite boring as there will be few other kids onboard, so no one to hang out with. And on many of the cruiselines, they won't offer as many activities for kids when there are few onboard.
If you want to take him on a cruise, consider a 4 to 7 night Bahamas or Caribbean cruise, or a short Mediterranean cruise. There's more likely to be other teens and activities geared towards his age group, which will keep him interested.