I'm sorry to hear you are facing this.
I had similar with DS at primary.
There was another boy - same age - who seemed to target other children. He would pick a victim for a few months then move on to someone else.
I knew about it as he'd been targeted friends of DS and their mothers had spoken about it.
The situation was complicated as the boy in question had quite a difficult home life and learning difficulties. The school was trying their best to support him whilst also trying to balance the safety of other children.
DS started to come home with injuries, far in excess of the usual rough and tumble in the playground and admitted this boy had been attacking him.
We wrote and spoke to the school. They did their best to keep them apart - the boy was watched at playtime for example. However, the incidents still continued, albeit less frequently.
DS came home one day with a huge lump on his head and very upset. He'd been pushed by his head into a wall.
Now I'm not recommending this as a course of action, but frankly DH and I had, had enough. We took DS to school the next day and sought out the boys father. We forcefully (but calmly) made clear that these attacks had to stop and that he needed to make that clear to his son.
It wasn't well received, but we held our ground and remained very calm. We explained this was assault and said if it happened again we would go to the police and we wanted to give him the opportunity as a parent to prevent that.
Thankfully it seemed to work - in so far the boy never laid a hand on DS again. What he did do was simply move on to his next target
.
I can only think that there is a real conflict were the bully has very real issues and needs support vs the safety of the children around them.