I have had experience of dealing with a school regarding bullying.
Firstly their anti-bullying policy will not be worth the paper it's written on.
Secondly you will never get the school to acknowledge bullying goes on.
Write to the school about every incident. Name the bullies. School must take action when you name a child. Remind them in the letter that they have a duty of care towards your child to ensure he does not experience (what they call) "negative behaviour".
Ensure that everytime you write to school you send a copy of the letter to someone else - i.e. Chair of Governors or LA if it's a serious assault. That way school cannot deny receiving the letter.
Insist that every incident is investigated and recorded in your son's school file. For serious incidents make sure they raise a Serious Incident Report - a copy of which should be sent to the LA.
In this way you have a documented history of the bullying and the steps you have taken to get school to address it.
Keep a diary of all teh incidents yourself - what happened, what action you took, who you contacted, what their response was.
You can then take the matter to the Local Authority if your child is continually bullied and school has failed to respond. You can also present this evidence to a solictor with a view to bringing a case against the school at a later date.
Every child deserves to be able to attend a safe, supportive school. Your son is being denied that right and it will impact adversely on his education.
Best wishes