Thank you for the clarification. Yet, I need to state again that your Queen Mary 2 voyage is not a cruise...it is a crossing on an ocean liner, and there is a big difference.
When you say you "have never done the formal nights," what exactly do you mean? I suspect that you did not go to the dining room, but did you also skip the evening shows, bars, casinos, and other public rooms? If that was the case, you'd be OK. But, if you wish to attend these on QM2, he will need the jacket and tie. He can even wear the same suit jacket all four nights (two formal, two informal), if he is concerned about packing. Otherwise, you will be missing out on a lot that the ship has to offer.