Should my husband take a long pair of trousers in addition to the dress shirts he prefers to wear?
A pair of slacks is a good idea for the nicer restaurants.
Yes, especially if you plan to go to medium or nice places for dinner. Shorts are more common than in the past but are still not going-out-to-dinner attire.
I'm just curious - does he wear dress shirts with shorts? What is the alternative if he didn't bring the trousers?
There's not a lot of context here, but was your husband seriously considering traveling to France without long pants?
As K mentioned, it would help to have a bit more information. Was your husband not planning to take any long trousers?
I'd recommend two pairs of "nicer" trousers, as some restaurants may frown on those wearing shorts. Having two pairs allows one pair to be available when the other pair is in the laundry.
Casual Khaki's or similar are practical for most city travel situations for men.
In order to travel as light as possible I have resorted to traveling with just two pair of convertible pants from Clothing arts.com, this way I am able to actually take two pairs of pants and two pairs of shorts. I wear one pair and pack the other, since they are made of nylon I am able to wash them in the sink and they are dry by morning. I've worn these pants with a nice long sleeve shirt into elegant restaurants without any issues, I even pack a tie in case I have to look a little more formal. I usually pack five shirts including the one I'm wearing and I'm good to go for a 14 day tour of Europe. I am not affiliated with Clothing Arts, just like their products.
Two pairs of pants always go with me on trips. Convertible pants are butt ugly in my opinion. I also pack shorts that double for walking and swimming. P.S. I travel very light.
Edit: My very lightweight pants (with two zippered pockets, one behind a snap enclosure) are nice enough to allow me entrance to restaurants nicer than I am willing to pay when traveling.