You only need a dinner jacket and black tie if you decide to go into the Main dining there will be
one Formal evening. In the windjammer restaurant you can wear anything.
There will be 2 formal nights on a 7n cruise. You will find most will be in black tie or at least a suit. You have to be correctly attired to eat in the main dining room that night other wise it may be embarrassing when you are turned away.
No he doesn't have to have one but many will be wearing them or a suit with shirt and tie. To opt out of the formal nights would be a shame if you have never done one and that is what you would have to do if you don't have reasonable attire. If you've a Macro in your area they have a very good offer on at the moment for dress suits. Whatever you deside....enjoy, ... Neil.
I'm sure he will enjoy wearing the DJ, but a shirt and tie will suffice for formal nights without you being turned away from the dining room' - thats what I'll be wearing on our VOS med cruise in a weeks time. The dress code is 'suggested'.