My 9yr son has had a head twich for about a week now, what's the cause?

Likely a tic: At that age this is usually a transient (less than 6 mo) tic. These are thought to arise from how antibodies to a past infection create a short circuit to motor nerves. It can vary in intensity and the kid can sometimes supress it some, but has no real control over the process. Most fade over weeks. A review with the kids doc is worthwhile.