What your brother really needs are three things. Exposure, Acceptance (even if you fake it!), and Guidance.
Exposure to the community he's chosen to join. For example take him to Mardi Gras, let him hang around gay people his own age etc.
Acceptance. He's your brother, everything you like together can still be done. If he let's you talk about girls, let him talk about guys. As long as he trusts you enough to spill his guts once in a while without fear of violence verbal or otherwise, then you get to do the third.
Guidance. This is where you make sure he's not being taken advantage of. There are a lot of perverts that he could meet in chat rooms etc, and later on in life there are the same relationship problems that straight people have. Also if you can drum it in his head that just because you can't get prego with a guy, you still have to wear a condom for other reasons.
Also make sure that he can change his mind and it's okay to like girls even though he said his gay, you can't get it right all the time ; ) That's usually better to say if it actually happens.
All of these can be done over a lifetime and if he's gay for the rest of his life then eventually you'll just have to accept it and move on.