It sounds like he wants you to dress in a way that other guys won't look at you, so he isn't insecure that you might find a 'better' guy.
I NEVER tell my girlfriends how to dress!! THEY CHOOSE WHAT THEY WEAR!! Who am I to tell them how to dress? Is your guy super-religious? Maybe that might be why!!
Do you want to be controlled, and dominated, and told what to wear?
He sounds like someone you should RUN AWAY from!!
When I am with a woman, her choice in clothes, her sense of fashion, and what she likes to wear is a part of WHO she is, and defines her!!
Yeah, I dated a 'librarian', and she wore long dresses or skirts, but she was TOTALLY different, when we were alone!!
Why would you let a guy tell you what to wear? You can wear whatever you want, and if he doesn't like it, get rid of him!!
It would be different if he wanted you to dress like an innocent 'virgin' for a sexual role-play encounter, but it sounds more like he just wants to control you!!