I don't think it's so easy to separate what is "real bullying" from what is "just kid stuff," because honestly, if your son doesn't want to go to school for weeks, something is happening that's really affecting him. The key point here isn't whether it fits some technical definition of bullying, but that he's suffering and you need to act.
First, talk to him without pressure, but then you need to document what's happening. Ask him specifically: what do they say, when does it happen, who are they, if something started it. Write down what he tells you (dates, names, words they used). Then contact the teacher or principal, not as someone going in angry to complain, but as someone saying "my son told me this, I'd like your help understanding what's going on in the schoolyard." Teachers usually see behaviors that we parents don't. If they say they haven't noticed anything, ask them to pay attention without telling the kid that you went to them (a lot of times things happen when adults aren't looking).
If after that things don't get better or the teachers say they don't see anything but your son still doesn't want to go, then yeah, consider talking to the school's discipline office or psychologist. And at home, keep the doors open so he keeps talking to you. Sometimes kids need to know that someone's got their back while things get sorted out.