What Causes Pain In Legs Along With Involuntary Movement Of Head In A Child?

My nine year old son has recently started nodding his head upwards like a twitch almost. He describes it as having a itch you want to scratch and try not to until its so bad you have to itch it. He also has been having pain in his left leg for a while but the head swinging has only been for about three days. At first I just thought it was from sleeping while holding his head up with his hand. Any ideas what could be going on?