HELP! HELP! HELP!HELP!HELP!?

Just tell his friends he isn't gay, simple as that. If they don't believe you, tell them all the information you currently have on the subject. then they might believe you. especially the thing that your brother's friend said "I like the way your a$$ moves when your running". Or confront his friend and make him tell the truth. But be sort of a scary big sister in that situation, otherwise he won't suck up to it.