My favorite is Keeping You a Secret by Julie Anne Peters. It's about high school, falling in love for the first time, and deciding when to come out and how much your decisions will be influenced by your friends/significant other. It is, however, about girls, so it may not be perfect for your brother.
A group of books that I see in the same place at Barnes and Noble where I got Keeping You a Secret that looks good is by Alex Sanchez. Rainbow Boys, Rainbow High, and Rainbow Road. I think he also wrote some others. I haven't read them, but I have flipped through them and they seem interesting, I just didn't want to buy something that obvious. (Was still living with my parents.)
I really think you're an awesome sister!

They're mostly non-fiction short stories, not novels, but David Sedaris's work frequently talks about his experiences knowing he was gay as a kid growing up in the South. They're funny and humanizing. You can also google his NPR segments where he reads many of those stories. If you want a good book to start with, try "Dress Your Family.." Good luck.

There is a book by James Baldwin called "Giovanni's Room" about a man who is struggling with questions of his sexuality. You might try that one.

Geography Club by Brent Hartinger is a great young adult fiction novel about a group of kids who want to start a GSA in a non-gay friendly school. To get faculty and administrative sponsorship, they petition for a club called "The Geography Club," thinking that no one will try to join under that pretense, and only those who know its true purpose will show up.