If he doesn't have a jacket on, Mickey and the Pluto police aren't going to march out of the kitchen to drag your husband to the door and throw him out of Lumieres. However, he will be in the clear minority if he is just wearing a shirt.
On our 3/23 Magic cruise, our rotation put us into Lumieres twice (once on the Thursday semi-formal night). If I had to generalize, both nights something in the range of 80-85% of the men in the dining room were wearing coats (with a noticeable minority -- including stuffed-shirt types like me-- even adding a tie). Thursday was definitely a little dressier, given it was ship-wide semi-formal night.
On both nights, while the guys just in shirts didn't blatantly stick out, one could definitely notice a subtle difference: the parties seated at tables with the jacket-less guys tended to be less well dressed in general, e.g. the women and kids weren't as "classy" looking either. Did they look tacky? No, but then again a little less tastetful, given all the "better dressed" people surrounding them.
Bottom line: formality of dress and "what is appropriate" is a highly, highly, highly subjective thing that comes up again and again on this board and never really gets resolved. So what do you do? Just go what you are comfortable with. If hubby doesn't want to wear a coat, fine, just realize you may or may not feel entirely comfortable with that once you are actually sitting down to eat in Lumieres.
