When it comes to pediatrics, I am just not equipped to be completely accurate in my answer to you. With that said, if you noticed a friend doing what your son is doing, I should think you might be concerned. An EEG can tell a neurologist a lot of things about the brain activity in a person, to include your son's head nodding perhaps being a sign of seizure, which would show up on the tape readout as way out of normal. I'm not saying he has seizures, that is merely an example of a whole raft of diagnoses.
The EEG is relatively inexpensive and very easy to do, it is noninvasive, and it is most helpful in narrowing down the possibilities of why your son is doing this. It is important to find out, because let's say he does have seizures, well, the causes can be many, but if he winds up being epileptic, by golly he has got to be on medication to control them, because it's not good for the brain to have a seizure, plus it's very hard on a child, even though it doesn't appear so on the surface.
Hope this helps a little. You could try posting over in the pediatric forum at this same website, could be some mothers over there will be having similar problems, and you'll get more replies. You could also do a search in that pediatric forum for "seizures," and a bunch of posts will pop up, so you could learn some stuff from those.