This could be a serious bullying problem, or it could be young children's lack of impulse control, teachers lack of attention to children at play time, or a number of other issues. It is hard to know if these behaviors are actually "bullying", and I think the best place to put most of your effort into is your child.
Knowing how to protect oneself is indispensable; but it is also not very useful if you don't know when to defend yourself. When you teach him to defend himself, make sure you include clear guidelines/rules for how/when to apply these skills. Of course, I also make it clear to my children, that when you really feel your safety is threatened, and there is no one around to help, all rules are off regarding the safety of the aggressor (in fact, the rule is to inflict as much pain as quickly as possible- but this probably does not apply to the 3-5 range).
I would think it is unlikely that he is going to be seriously or permanently injured (physically) through bullying in kindergarten, especially while at school, but it could lead to long-term problems and setbacks socially, emotionally, academically, etc. It is important to address this problem quickly, but it is not likely something that will just disappear after a talk or two with your son.
I think the best thing you can do for him is to:
- Boost his confidence
Get him to believe in himself and develop his own sense of right and wrong. With a strong sense of what is right/wrong, he can better decide when he is being mistreated, and with a high level of confidence he is more likely to act in his best interest without overreacting. A good way to do this is through roughhousing with your kid. See this Art of Manliness article about the benefits of roughhousing.
- Help him develop his sense of right and wrong
Hitting and biting aren't wrong. Hitting and biting hurt, and hurting people for fun, or to get your way, or to show your better than them, or to get back at them, etc., is wrong. Hurting someone to keep yourself safe is not pleasant (shouldn't be pleasing), but is perfectly acceptable. I also like to make it clear to my older child (not the 3-year-old) that not everything we learn in school is actually useful, and not everything we are told in school (or by every adult) is actually true.
Around here, we have some "formal" roughhousing games in addition to "free" roughhousing. The games are:
Flick or Kick
This game involves one person asking the other (the rules should be explained first) a question, "Flick or kick". Depending on the answer, the asker either flicks or kicks the other person. You then switch roles.
Punch or Munch
After having the first explained, you can probably guess this one. The initiator asks "Punch or munch?", and depending on the answer, either begins punching or munching (pretending to bite) the other. This one tends to be messier as drooling (but no actual biting) frequently occurs.
General rules of roughhousing in our home:
We don't intentionally hurt others
Actual biting, hitting with inappropriate objects, hitting too hard, or hitting vitals of any sort immediately ends any roughhouse game.
We keep our bodies under control
If one person starts to get upset, too aggressive, scared, etc., we take a time out to calm down. This is normally snuggly time.
The games sound brutal, and are in concept, but they are a lot of fun, and no one has gotten (seriously) hurt. Check the article on roughhousing and follow it up with some of the supporting material and searches of your own. It might be one of the best tools you have for addressing this type of issue.