It is a complicated situation, and a lot depends on school administrators. Usually they'll cave if you have an attorney contact them directly, threatening lawsuits and publicity. There is always the chance that the bully's parents are even worse than their kid . . . that is learned behavior . . . and going over there could escalate unexpectedly, possibly resulting in criminal charges.
My best advice is to tell that kid that, from now on, every time that bully comes within 3 feet of him, he should just pre-emptively punch him right in the balls . . . no warning. He will probably get his ass kicked a few times, and might even be suspended, but eventually he'll be left alone. He needs to learn that he can't count on other people to protect and coddle him.

