If DH never wears a suit or sport jacket I would not buy one just for this cruise. You are talking about wearing it for 2 hours on formal night. I don't know the dining room policy on the ship you are going on but if you can't get in the main dining room with just a shirt and tie there are certainly other dining options available. He will not be the only person on the ship that does not have on a suit that night.
7 Norwegian: Sky, Jewel, Dawn, Epic, Pearl, Breakaway, Getaway
9 Windjammer: S/V Legacy X3, S/V Polynesia, S/V Caledonia, S/V Diamant, S/V Sagitta X2 S/V Mandalay