If you don't mind me asking... why is it the brother does not (or did not at the time of the post at least) know that the test was positive? Are you saying he don't know he did the test in the first place? How on earth did they make a blood/saliva/urine test that he wasn't aware about? Did they lie and say that the test came back negative? O.o
It's a hard question but i agree with Hurricaneclaw. talk to his parents. Descide with deep conversation who should tell him; his family and/or you. You're right in thinking he shouldnt be in the dark on this but, with that said, find out if you should tell him or his family. Some people may feel it a touchy subject for even their BEST of friends to know...
I think is not your duty. Even of you are a good friend, this kind of things are a family thread. I suggest you to talk with your friend's brother and his parents about the moral struggle you feel, you'll find a solution. Be strong, i wish the best for your friend's brother.
I don't believe it's your place to tell him. If your family is close with his family, then perhaps you could talk to his parents about it, or ask your dad to talk to them. It really seems like it's up to them to let him know. You can be there to support him when he finds out.
It is best that you let his parents tell him. You can tell him, but then he might get mad at his parents for not telling him, and right now they need peace among themselves so as to give his brother all the support and love that he needs. What you can do, is talk to his parents about telling him as soon as possible - it isn't right that he doesn't know yet. Other than that, you can only pray for them, and support your friend as much as you can.