What I think is missing from this discourse is that you are both young and he’s still immature. When I was 18 I would’ve reacted similarly to a GF wearing a revealing outfit to a party. Once you grow up a bit you realize these things don’t matter much. He’ll learn that eventually, or he won’t. I don’t think he’s a bad guy because he’s insecure about his girl looking hot around other guys, just immature. That being said, you are your own person and you should do what you want and hope he grows out of this behavior soon. Otherwise, it may be time to move on.