What would have happened to your little brother if he had fallen off the bike instead of being able to pedal and get away? That dog could have killed him, and may do so next time!!!!!!!
Please call animal control and the police, as others have said. I love dogs, but also agree that a dog that attacks humans is not a dog to have around. Way, way too dangerous. I hear on the news all the time about people being attacked and sometimes killed by dogs. Don't be another statistic. Take care of it while you still can.