Hi,T.. It sounds like your brother is a little immature, no criticism intended, but he sounds insecure about himself, that his masculinity depends on being able to have worry-free sex, he has no sense of responsibility for using protection, and he has a totally sexual view of relationships -- he's going to have to get over some of these before he can cope with the possibility that he may have herpes.
He probably knows that living with herpes or any other condition that "cramps my style" is going to force him to mature some more, and he may not be ready for that yet. I think the only thing you can do is to try to be supportive so that he doesn't think he is all alone in the world, at least until he gets the results back. He also needs to contact the girl he was having unprotected sex with.
Whatever his diagnosis, this could turn into a positive thing for him. #1, he should become more responsible for protecting himself and his sex partners, and #2, he should become less likely to have a whole lot of casual sex.
In the meantime, I will pray for you, my dear, for your healing! I hope that God's love and healing touch you deeply, now and forever.
Peace,
Syl