Hey VT.
I have been doing a bit of searching, and haven't found much.
The first thing is, not to start worrying until you have something to worry about. I have known some guys in the armed forces, and they take care of people. They may put them in dangerous spots, but they get them out and they have damn good doctors.
The next thing is that they will protect your brother's privacy, and this is a good thing. They will not publish his details just anywhere. This is important because he is still stationed overseas.
I read through some articles on the subject. They indicated that the US State Dept will try to contact family members 17-24 hours after being notified, although it can take up to 72 hours. Am I right to assume from your post that someone has been notified, and you have found out in the form of a rumour?
If you cannot pump the person you heard it from for more details, you could contact the base at which his division is normally stationed. Whilst they may not be the correct people to talk to, they may at least be able to talk you through what is the correct procedure to find out. You may also need to provide some kind of identification as to who you are. Be prepared to do this.

I hope all goes well.