if i was a girl.... id wear a tight corset then complain that all the guys stare at my boobs. na to be fair i have no idea what you should wear why not just ask him...

Wear something you feel comfy in! And something you know you look good in
You can never go wrong with a simple (usually black) dress but pretty accessories!
(hope I helped!)

I agree, you can't go wrong with it. It's classy and elegant, yet something that is acceptable with whatever he has on.

Shy has a point. That's a very good option.
Another thing you can try is a simple dress with a fancy coat (it is winter after all). That way, even if he doesn't dress fancy, you aren't over-fashionable for him and you can just take off the coat later to make him more comfortable.
And if he does dress fancy, you still have that extra flair to you, just like how the accessories would do with the other suggestion given. Balance yourself out, feel comfy, don't let yourself have a headache over it :P