Why wont my boyfriend tell me why he thinks id brake up with him over crossdressing?

Most likely because he's had many people not accept him in the past. Whether it be family, friends, or girlfriends. Keep reassuring him that you won't break up with him. It's probably just hard for him to accept someone and let his guard down. Keep being his best friend first and foremost, and he'll eventually accept that you won't break up with him. :)