You shouldn't have to change yourself for a boy.
A boy who truly loves you, wouldn't threaten to break up because of what you wear or what you don't wear.
But relationships are all about compromise, if you truly want to make it work with your boyfriend maybe you should ask why it's important for him, that you wear makeup and fashionable clothes. Is it a constant thing he wants, or dressing up once in a while? You could possibly use it as leverage for something that bothers you about him and maybe you both can compromise to make eachother happy!
But you need to decide if that's someone you want to stay with. Because he sounds superficial.