You need to ask the school if they have an anti-bullying policy.
They have to by law
Here's an example:
http://www.kidscape.org.uk/assets/downloads/An tibullypolicy.rtf
Note that it lists signs and symptoms so the fact that your son has not reported the issue does not absolve them of all responsibility.
Talk to them about their policy, how that works and why it is not working in your son's case.
Now it's not clear from your post whether this is an isolated incident -possibly even normal rough and tumble or the culmination of months of serious intimidation and victimisation.
You need to be clear in your mind about this, incidents dates etc. etc.
But keep it calm and in perspective.
At the very least they need to talk to the children involved so that they know they cannot just get away with unacceptable behaviour