My kids go to every formal nights. Boys wear suits and ties (11 & 20) and girls wear dressy dresses (13 &16).
Celebrity Century 1/09
Carnival Destiny 10/07
Carnival Destiny 9/06
Monarch of the Seas 3/06
Carnival Inspiration 8/05
Carnival Miracle 4/05
Carnival Inspiration 4/04
Sovereign of the Seas 4/03
My grandson (who is 14 years old) always goes to formal nights, when he was very young, we dressed him in tailored shorts and a nice shirt, now he wears long dress pants, a long sleeve dress shirt and a tie (no jacket). He is dressy casual but very comfortable. I've seen a lot of little boys wearing suits but it isn't necessary. I see a lot of men that don't wear ties and jackets, just dress pants and nice shirt. You'll find people that go way overboard on formal nights, it isn't necessary.
You will find all kinds. There will be some very dressed up and others that will have nice khaki's and nice shirts..no ties or jackets. I'm sure this is what my son will do. My husband doesn't even want to do the suit thing. Feel what you're comfortable with and have fun!
THe first formal night is always the more dressier of the two.
Most teen boys will be in dress pants, dress shirt and tie, dress shoes. If they have a jacket, bring it; if not, don't worry about it. On casual nights, most will be in docker type pants (khakis) and a polo or button down shirt (hawaiian type or print).
PS........right after dinner, most of them will change back into their shorts
While not many wear a tux many do wear coats and ties and others wear the dress clothes and ties. If the boys are old enough to appreciate it having them wear a jacket and tie or even a tux is very good for them to let them know that they are almost men now and should dress like men do on "formal" or dressy occasions. Not a bad lesson for them.
On the cruises I have been on, I would say a majority of boys wore either dress pants, shirt and tie or khaki pants and dress shirt. Some wore a suit or tux, but it wasn't the majority. You will see such a wide variety of dress for both children and adults that you really can't go wrong as long as you make an attempt to look nice.