When we took our first
DCL 8 years ago it was a 7 Night, my sons were 16, 12 and 10. We had a formal night, semi formal and cruise casual, plus pirate night.
From what read on DIS Boards and DCL website, they dressed appropriately.
Formal night-Dress shirt, ties and khakis. Semi formal-collared button down, no tie with khakis. Cruise casual-collared shirts (golf shirt) and khakis or dressy shorts.
Since then, they have only gone on 3 night with cruise casual, but they always wear a collared shirt, khakis or dress shorts to dinner. They all attended private school where they had to wear shirts and ties to high school, so they have dressier clothes. And they are used to wearing nicer clothing. Plus, they all like to look nice when going out to dinner etc. So, no problem dressing up a bit for dinner on the ship.
I have noticed over the years and on shorter cruises people tend to be a little too casual. Tee shirts and shorts to dinner. Now you don’t have to go all out, but a golf shirt and neat shorts should be the minimum for men and young men for dinner on the ships.
My DH wears a suit everyday, so he would prefer not to have wear one vacation. But he all wears a collared shirt and khakis. And when the occasion calls for a tie and jacket, no problem.
Ladies dress is easier, sun dress, nice slacks or skirt and blouse. To dressier on formal night.
You will hear from others. If your son is very young, then nice shorts with a collared short can be acceptable. But a pair of khakis is a great thing to have.
Good luck.