Your husband will love Freestyle cruising if he does not want to dress in the evening. They still do have formal and "theme" nights, but they are restricted to certain restaurants. I liken Freestyle cruising to like going to a resort somewhere.
One of the resort's restaurants may be formal, the next a themed casual restaurant, the next a buffet, and so on. There may be a formal party happening in one of the ballrooms and then an informal party in another.
If you pass through the lobby of the resort you may see folks in shorts, someone in a tux, others in suits, maybe some in slacks and golf shirts and so on. Ditto with Freestyle cruising. Give it a try.