OP I hope you get it sorted. One piece of advice, keep on at the school. don't let it drop.
My ds was also bullied by a child with sen. Started as name calling, escalated to full on assault. Including things like being pushed down and having his head stamped on, having his head smashed into a brick wall. The school were always 'dealing with it', but nothing ever seemed to be actually happening. The child would taunt my dc that he never got punished and would laugh at him about that. It would stop for a few days then start up again. They simply did not take it seriously, and their excuse was this child has 'issues'
I slowly watched my DS go from a happy, bright bubbly, easy going 8 year old to a quiet, subdued 11 year old with zero self confidence and depression who would get anxious to leave the house. I wish more than anything I'd of been more forceful from the get go. It wasn't until my ds said he wanted to die that I really realised the full extent of what he'd been enduring at school.
It made no sense that the school were not punishing this behaviour because sen or no sen, a child needs to know what is acceptable and what isn't. And they were not being given this message, just allowances and excuses.
Forward to secondary school and it continued. Except they were not half as placid with this kid as at primary. It was stamped out pretty damn quick. The kid in question went on to assault a young girl (because she looked at him weird) ended up expelled and is now in juevenille jail (whatever thats called officially) for further assault on an old lady.
So ask the primary school, how is their allowance of this childs behaviour acually helping him. To me, my ds primary just seemed to want to brush it all under the carpet until he was no longer their problem. Destroying my ds and other dc in the process.
I'm sorry for parents who have violent dc, sen or no sen, but when they are physically assaulting other dc they need to be removed and actually properly helped, because its not going to just vanish as soon as they hit teenage years.
My DS is now 14 and so anxious socially, he keeps his head down and avoids any situation he can. He is no longer that happy lad who was full of jokes and spirit and fun. If I could ever meet this other kids parent I would say 'did you know what your son was doing?' and 'why the fuck then did you allow him just carry on'. It kills me every day that I didn't realise just how bad it had got until it was too late.
Don't let them fob you off.