How do I explain this to my boyfriend without coming across as a b?

We have a little outing this weekend with his family. I asked him what I should wear (it's a barbeque) and he told me to wear a pencil skirt and button up! I think this is being way overdressed and he even told me everyone else will dress casually. How do I tell him I want to dress the way I want to dress? He's making me feel a little like his barbie doll. Or am I wrong? Is he really asking all that much and should I just do as he asks?