J.S.
Hi R.,
My son is 9 1/2 months old and he used to do a similar thing. While sitting and playing or even while on his belly he would drop and raise his head almost like he was headbanging. He would always smile and be happy while doing it, but I wondered if it was a voluntary movement or a neurological thing. I would distract him from doing it - play with him, bring out another toy, change his position and he would stop. So I figured it was a voluntary thing - something new he was just exploring and having fun with because he learned how to do something new that got our attention. I was more afraid that he could hurt his neck, so I always distracted him from doing it. He doesn't do it anymore. While I wouldn't worry about your son, don't hesitate to talk to his doctor about it. As a mom, you have instinct that you need to listen to. Good luck :-)
~J.