It’s your relationship, not your mom’s. She should want you to be happy, make it clear to her that you want to be with him, stand your ground. She’ll most likely be more upset that you’re keeping it from her than she will be that you’re with him.
Just my opinion, but maybe just spruce it up a bit. I mean don’t wear something you’re not comfortable in or something that you just don’t feel like yourself in but go for a more feminine look something that makes him look at you and realize he doesn’t just want to be your friend. Show some skin.