Apologies if you tried this last time but I have had similar issues and this worked for me.
Ring the school again and ask to speak to the head or deputy responsible for dealing with bullying etc. Take the name of the person you have spoken to and send a follow up email to confirm the contents of the conversation, copying the email to both the Head of Year and Form teacher for your son. Note e.g. that you spoke to X as recommended by Y as being the person responsible in these situations. Then note your conversation with X or the fact that you were unable to speak with X and remind them of your contact number plus the fact that they can contact you at any time to discuss the matter.
If they do not reply, ring again, ask if you can speak to X tomorrow and again by email note that this is a second call and that no one has responded.
Keep going until you receive a call or email. Be unfailingly polite and calm but relentless.
Each time you speak to someone, send a follow up email, copied to the form teacher etc.
In my experience, the school will have to respond to you or they will have an unhappy time explaining themselves to the governors etc if they fail to respond.
You are not entitled to know anything about any other child involved, make it clear that you understand and respect that but that a strategy needs to be put in place to protect your son.
I did this and it worked to get the frankly lazy and complacent staff to stop my DC being bullied. It taught my DC that bullies can be stopped and that things could be worked through. I admit we changed schools at the end of the year though because I was so disappointed at the amount of effort it took to get the staff to do their job and I just didn't trust them with my DC anymore.
The new school is fantastic and the staff are really proactive. You absolutely don't have to put up with bullying.