This is why I am going to emphasize to my son that if someone begins to bully him, the sooner he corrects the bully's assessment of him the sooner it will likely stop. This may include violence (with many caveats) if necessary, as frankly, it's the only language they often understand and respect. I am not happy that this is the case, but this behavior appears to be a universal part of the human condition. Based on my experience, adult intervention or even supervision cannot be counted on, so it will be something that eventually he will need to learn to handle himself.
Bullying in it's most common and less extreme forms appears to be a way of reinforcing group conformity and norms, as it's often directed at those who are different in appearance or behavior. By calling attention to this with name calling or teasing, they publicly demonstrate their own in-group status and as a side-effect also pressure the target to conform to group expectations. Over time, this can be just as insidious and damaging even though violence may not be always involved.
In the most pathological form, you have a person that has a lot of internal uncertainty around their own position in the social hierarchy (which actually may be quite high already and part of what makes this pathological) for which bullying is a way to assert their dominance over another. These are the bullies that we all know and which seem to fixate on a particular target or group, and are the type I alluded to in my opening paragraph. They are not necessarily spoiling for a fight (that is something else entirely and distinguishing the two can be difficult) but rather have identified their target as being someone weak or low status enough that they will not fight back nor will anyone rise to defend them. However, if the target does fight back and even if they lose, if this happens consistently then that is often enough to update the calculation on their end about whether the benefits outweigh the costs (after all, they could eventually lose the fight or at least it could be close enough that they don't win convincingly which is almost as bad) and their attention will shift elsewhere.
There is also a greater likelihood that others may stand up for you if you stand up for yourself. It is another tendency of human psychology that I have observed that those who do nothing and essentially "take it" are viewed with contempt even if those observers also disapprove of the bullying. There seems to be some general undercurrent of disgust with perceived weakness, but this is perhaps rooted in some sense that the risk of intervention is less likely to be reciprocated. After all, if the victim doesn't appear willing or able to do this for themselves, why would they be expected to do so for someone else?