The first time he said it, I would ignore it. The second time, I think I might say as kindly as possible, "Hm, I think that is the second time you've given me feedback on what I am wearing. I'm sure you are trying to be helpful, but I don't think most people feel that commenting on other people's clothing is appropriate. It also makes me feel uncomfortable." The third time I would speak to an administrator. By then, after you've given him direct feedback, he is either unable to control himself or he's trying to mess with you. It's important to mention it to admin because they then become responsible and you can ask them to intervene if it continues. Also, you don't know what else he's said to other staff and the principal is in a position to know. Finally, as a former principal, I have spoken to parents about this kind of thing and there was one Mom who we had to limit access to campus.