It depends on the couple. I really couldn't have cared less what people wore to my wedding. I didn't even pick out my bridesmaids clothes.Both my MOH and my "Brides-man" went shopping together 3 days before the wedding and she picked out both their clothes because I didn't care and they are procrastinators.
My SIL on the other hand had to approve every thing that anyone picture worthy was going to wear.
If you're close enough to the couple that your DH is in the wedding I'm sure you could call and ask. People called me all the time to ask what they should wear, I didn't think anything of it.