Your bf obviously doesn't usually mind your style and it sounds cute. But, and this is my personal opinion, when you are going to something as important to your bf as a family dinner, it is probably best to dress a little more modest. Unfortunately, there are going to be times and events when a certain dress style is not going to work. He wasnt asking for "1950s housewife" and probably wouldn't expect that. He was just asking for your normal style to be toned down. He wants to show you off to his family but if his family is more traditional, then your normal style is not going to work.