I attended a very small school in Columbus, OH. I know of at least 4 GLOs that had openly gay and/or bi members while I was there. It wasn't an issue with anyone. They brought dates (guy/guy or girl/girl) to their functions and no one blinked-they just had fun. There was a relatively high gay student population in general, I think, and everyone had a few gay friends anyway. On top of that, all of the GLOs were small (20-40 members) so people knew each other really well.
I think a lot of it does have to do with the environment your campus is in & the students involved in greek life. Columbus has a large gay population, and some of the best clubs were "gay" ones. Our students were exposed to it all of the time.
The faculty & advisors tended towards the liberal in general, even though the campus was rather conservative. So there was a lot of support for minorities of any kind.
Back to the original question- I think your friend should come out. These are his brothers... why should he lie to them? He might do well to come out to a few close friends in the chapter first, so he can gain some support before coming out to everyone.