If you are getting all dolled up for Formal Night, it would be fun to rent a tux onboard for your son........that way you don't have to mess with getting one, packing it, and taking it back. It would just be in your cabin when you got on, and you'd leave it there when you went home. It would be great for a family picture, and he would feel pretty special.
Just a thought..................... (of course, you know your son best......)