I have a motto in life -
Never upset anyone if you need their co-operation.
Bear this in mind when you talk to the Head.
You want a resolution, but you have no actual power in this situation - so you need to be co-operative from the start, and keen to hear both sides of the story.
Of course as a parent, your instinct is to steam in and raise hell, but that will simply underline the impression that your child is a rebel - look at his background.
Be calm, listen, and then see if it can be resolved. If you need to go in hard afterwards, the option is there, but if you start off shouting, you may look foolish if they agree with your side of things.
Good luck - please let us know the outcome.