Reading this post, all I can think is that people are horrible, and that you have a really amazing brother. Did he have a rough time in high school? Sure. But really, who doesn't? Its not your fault. AT ALL. You are who you are, and your brother was strong enough to take what hand he was dealt because he loves and respects you for who you are. That's what family should do, and what a good family does.
Again, its not your fault. Its the fault of stupid, ignorant, sexist, bigoted, etc etc etc pigs. Your brother was smart to realize that. But if he can stand up through it all for you in high school, which may be the hardest time in most people's life because its all about striving to fit in, and more peer pressure than pretty much any other time in life that I can think of, it goes to show the kind of man he will be the rest of his life for you.
So... Like Laurie said, tell him you love him, and thank him for supporting you through thick and thin, and its something you'll always cherish.
Yes, you can feel sad he went through that on your account, but never feel guilty for it, because you did nothing wrong, it was the idiots who should feel guilty. And who knows maybe one day they will. (Watch, they will go get a job in the workforce, and even if their boss isn't trans, maybe that boss will be gay. That'll show them, if they haven't learned their lesson yet.)