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.
56 Carnival: Mardi Gras, Ecstasy X2, Fascination X6, Celebration, Elation X5, Glory X5, Fantasy X4, Valor, Holiday X3, Imagination, Liberty, Spirit X2, Freedom, Destiny X2, Splendor X2, Dream X2, Conquest, Legend X3, Victory, Sensation, Paradise, Inspiration, Magic X2, Triumph, Miracle X2, Breeze, Sunshine X3
7 Norwegian: Sky, Jewel, Dawn, Epic, Pearl, Breakaway, Getaway
7 Windjammer: S/V Legacy X3, S/V Polynesia, S/V Caledonia, S/V Diamant, S/V Sagitta