What do you do if the best man won't wear a tux to a wedding?

Me and my boyfriend are getting married. The best man who is also my brother and my boyfriends best friend refuses to wear a suit or a tux. This is my boyfriend's first time getting married and my 2nd time I have never worn a wedding dress and I want to. We are planning on a church wedding my boyfriend wants the tux and wedding dress the whole 9 yards but my brother refuses to wear a tux.My boyfriend want everyone to look nice he thinks it would look stupid if everyone was dressed nice and we have the best man in a t-shirt and jeans what do we do?