As many have said here bullying is a complex issue that isn't always easy to pin the source of the aggression down, or an easy way to end it. Someone said that the bully should respect your brother, but that doesn't necessarily mean a fix either, since the kid may respect him and thus hate him even more because he knows that he's stronger willed than he is. There's always a way to work it so he'll come back at your brother twice as hard regardless of how you choose to approach it.
However, there are good avenues you can take that are a valuable in any situation. If you can get the kid on audio or video being psychologically abusive, or threatening physical violence (whether acted upon or not) you may have a case against him to take his parents/him to court. Each state has it's own laws concerning aggression and bullying, and your state may have a law that you can utilize against the kid. The more evidence you have the better, if you get audio recordings make sure to record the time, date, place and people on the recording as soon as possible to get them right. If you can get witness accounts get them quickly and verify the persons contact information. Here's a website to start from to look into laws against bullying in your state:
http://www.bullypolice.org/
With regards to physical defense, it's always a good idea to be able to defend yourself, but it's not always a good idea to initiate the physical altercation. If the kid is good at sucking up to adults, he's probably going to be able to warp the situation in a way that benefits him and makes your brother look like a bad guy - especially if he never actually initiated any physical contact. In the U.S. it's not whether you were in the right or not or even if there's solid evidence for your case - it's how good your lawyer is and how you present your case. Even if you get him on tape threatening your brother and then your brother decks him in the face for it, there's no guarantee that the evidence will be permissible in court. Again, that isn't to say he shouldn't defend himself or to imply that you shouldn't bring the kid to court, I just mean to say that it's complex.
The ideal is to help your brother work towards a state of personal solidarity. By this I mean get to a place where he feels capable enough to withstand physical, psychological and emotional threats. This is a hard place to reach, most people never obtain it, but you can start down that path and help to make him less of a target. If you can get him to a point where he feels strong enough and capable enough to defend himself, he can start to feel like he's safe even when being threatened. That will make him less of a target and less fun to prey on. If you can get him to where he knows in his heart of hearts who he is and that he's a good person, that his choices are sound, he'll be less likely to doubt himself when under psychological attack. He needs to know how to be quick on his feet mentally, as pychopaths/sociopaths who prey on people in a social manner tend to be extremely intelligent and able to twist a persons words at a rapid pace.
The emotional side is even harder, since there's less logic and memorization involved. Part of it is just getting used to a situation through experience. My welding teacher used to say "The working definition of wisdom is experience", and it's true. Every single person is different in this area, no one set of directions will get him to a position where he's emotionally solid and unwavering during an assault, he has to find a way for himself, or at least try different ways that are offered to him. Some people do well with the military approach, I personally do well with the 'sedona method' of letting go of bad emotions on the fly, others don't. It may be worth talking to a psychologist to help him find ways to get there.
I am not a psychologist, counselor, or teacher of any kind. I honestly don't have a simple answer and the advice above may not be helpful at all. I hope your brother, you and your school system can find a way to put a cap on the kids agressive behavior one way or another. Good luck in whatever you try :thumbup: