If this is a sudden thing, I think you should try ignoring it. It is most likely a nervous "tic" that will go as suddenly as it started. When my daughter was 6, she started doing this thing with her head, it looked like she was a chicken pecking. It started suddenly, and got worse the more people called attention to it. The ped said to ignore it and it would go away. It did. Then at age 9, she started shaking her head back and forth for no reason. She often knew she was doing it, but couldn't control it. She was under a lot of stress at school. The ped again told me to ignore it, and that really, there was nothing they could do for nervous "tics". We told the teacher to ignore it and the guidance counselor worked with her on stress relievers. It was gone as suddenly as it came on, and lasted a few weeks.
If he is under any stress, I would work with him on some stress relieving strategies. If it continues more than a few weeks, then maybe get a speech eval.