Help!! dog aggression towards strangers

Hey everyone!
Im hoping someone can help us! We have a Kelpie x Lowchen 11month old called Mo, he shows alot of anger towards strangers (especially men) whether we are at home or out at a park, he growls, barks and goes all stiff and im really worried that he may hurt someone. He has had lots of socialising as a pup with all sorts of people and dogs. He is quite good otherwise! except for chasing my 3 year old when he runs and biting his clothes! I really hope some can help. thanks heaps!