There are normally three styles of dress on board conventional ships. Casual: sports shirt and slacks/trousers, Informal: Jacket (and Tie) with slacks/trousers. Formal: Suit or DJ/Tuxedo. These dress codes are upheld on P & O by 80% of the passengers. If you want him to be in the 20% or less then don't take a jacket. Personally I like to wear smart clothes and I take two or three jackets and trousers and loads of shirts and ties and dress as I would if I was going out to a good restaurant to dine. You do as you see fit.....Hope that helps .....Nei.
My husband took tux for formal nights, but wore smart pants and short sleeve shirts for the other nights. Some wore jackets but the majority were in shirts (long or short sleeved.
No informal nights on P & O Caribbean cruises - smart casual on 10 nights, formal (including one 'black and white') on the other 4. Short sleeved shirts and chino type trousers are fine for casual wear. Save the luggage space for your own stuff!