I know I'm late, but here are a few thoughts.
Also, the comment above about advice not always being applicable is correct.
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First of all, regardless of the specifics, let him know you are there for him. Reminding somebody that there is someone who thinks they are worthwhile goes a long way.
In theory, ignoring a bully sometimes works out, but this is not always applicable. True, some bullies want to see people squirm under their oppression and bemoan their fate. Others just want a punching bag, physical or verbal. Some lack the sympathy and/or are empathetic enough to see through a bllank expression.
There are ways to directly confront bullies, but they are rarely worth the costs. My strategy was to react in a somewhat "crazy" fashion, baring my teeth and saying confusing statements to convince the bullies that I was not worth the trouble in case I ever snapped. It worked, but required becoming a social outcast for a year or two. From what I've seen, physically fighting back can have similar results, especially depending on school policies. Fighting can sometimes be worth if things get that bad, but school policies often have their own punishments. For that reason, an ideal fight is one that only needs to happen once. If you need to fight a second time, you did not do it properly the first time. Think about the beginning of Ender's Game and turn back the violence by a notch.
Even if "reasonable authority figures" like the principal are often useless, there are still people to contact. It does not always have to be a direct solution either. Maybe your friend is by himself in History class. If you have another friend in the same class, try introducing the two. It can be a good thing for both parties. Maybe you know somebody on the handegg team that is not a jerk. You can ask him for his advice on the matter, since he would probably know things about the bully you wouldn't. In that last example though, make sure it remains confidential.
Also talk to any other people in your friend's life who care about him, including friends, family, favorite teachers, or whatever. If applicable, exchange contact info for emergency purposes. This might not always be immediately fruitful, but at least it makes a net to help your friend in case something bad happens.
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My personal story: I was bullied on a fairly regular basis in elementary and middle (junior high) school. There were a lot of reasons for this, and it was a number of people on and off over the course of eight years. What got it to end was that in high school, I somehow ended up making friends and aquaintances with people who had enough social standing to prevent some of this early on.
Part of this had to do with how my personality developed going into high school. On one hand, I was passionate about academic studies, especially history and literature. I also developed a sense of humor at that time that was very situational, and thus could be applied to any given audience. Not to mention that I had lost quite a bit of respect for the school system by this time. So when people saw me, they saw a geeky but (intentionally) funny oddball who was eager to tell students with Biology Class in the Afternoon what they needed to study based on the test I had in the morning. Hey, it helped their grades and their understanding of the topic, so why would I care? Teachers generally liked me because I was passionate about the subjects and did my best to keep the classes alive. If this involved thowing a sexual innuendo out as an answer to a question about Macbeth... well, Macbeth had far wose. I suspect that a few potential bullies were kept away by the number of peers who would rather I keep on helping with their work and making jokes in class.
One one or two rare instances where verbal bullying may have begun, I sabotaged it by improving it. I got insulted, but then gave the bully tips on how the same insult could be changed a bit to be harder on me and more fitting of the situation. It both made them laugh and created a "Not Giving a Fuck" atmosphere. I also think somebody mentioned sleeping with my mother, to which I asked if they had a lovely time.
It also helped that I had spent a decade learning how not to draw negative attention and had finally gotten decent at understanding human faces.
I was lucky.
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Thanks for caring about him.
Love and tolerate.
