The first thing I've got to say is that you're awesome! If you feel that Crossdressing is you, then that sounds like a great way to be supportive. But, it's also not essential. I think that talking with him would be a great idea; just present the idea that you would dress in solidarity, and see how he feels about that. I'm reminded of a friend to whom I've recently revealed my gender identity. It was the most amazing thing when he simply listened, as well as asking if I'm a lesbian and even was willing refer to me in a feminine way. The fact that I got all that from a friend who has primarily interacted with me as guys/bros is SO much better than anything I ever expected. So, I just want to encourage you to be that kind of friend, because in my opinion, the absolute best thing you can do is talk and listen in a way that legitimizes all of your friends feelings as well as his identity.