Speaking as a teacher and the mum of a boy who was also bullied at primary school, the best advice I can give you is to go back to the school.
But before you do that, keep a log of every incident (and I mean every incident) with times, dates and places. Make sure your son reports these to the staff on duty when they do happen. If possible, write down your son's version of events - a witness statement if you like. And note down which other children saw it happen.
Then go back to the school.
Explain the situation is still going on, 'talking' to the boys hasn't worked (rarely does in my experience) and ask quite forcefully what the boys' parents have had to say about it (not -have they been told?). Assume the school has contacted them - from the sound of it they haven't done so, but they won't admit that and you'll be putting them on the spot.
Show the log (take a copy first), and ask whether any staff have seen these incidents, were they there? If it's at playtime, what were the staff doing?
Most schools still try to deal with this in-house, ie without bringing parents into it - mainly because as you point out, they think the sun shines out of every orifice and will kick off big style if the school 'accuses' their child of anything.
Insist politely that this has to stop and stop now, otherwise you will pursue this as per the school's policy. A simple "I will take this further" will do.
Your son has rights, including the right to an education without fear or intimidation, and it is the school's responsibility to deal with this.
Failing that, tactics that usually get an immediate response are - write to your MP, threaten to take legal action, write to the papers.
Good Luck.