Don't get in touch with the parents (it's not only the Mum's responsibility either! Dads have a say)
You'll get 99% of them thinking that their child would NEVER do such a thing. They'll deny it.
Make an appointment at the school to see his form tutor. Demand that something is done...but first, contact the school and ask to see their bullying policy...all schools have a bullying policy and you can ask to see that...read it and then go to an appointment with his form tutor.
You will be armed with the knowledge of what they should be doing.
Are there any clubs at lunchtime in this school? Often these can be a real saviour for kids who struggle socially;
And finally, what is it about your son that's making other kids see a victim?
This is HARD question to ask a parent. No parent wants to see their child as a potential victim. But there will be something...something that they're cottoning on to and using against him.
The most common reasons are that a child might be quiet and shy which means they don't snap back when faced with a verbal or physical challenge...or that they're immature socially and other kids see that and use it.
Or sometimes it can be due to the meanest of all reasons, physical appearance (including clothing)
Do any of these fit your son? If they do, there are things you can do...I know people say that it's not the bullied child who needs to change...but kids are like animals and unfortunately, sometimes it is needed to change.