@shouldistop that's such a tricky situation, mine have both had similar scenarios. It will be a child with additional needs.
If it's any consolation, it was worse in P1 and improved as the child settled and school worked out the best way to support.
I would be advising DS to always make sure an adult is present if he's around/playing with the little boy so your DS will not have to deal with behaviours himself. It's not his problem to deal with this.
And yes I would be phoning the office and asking for teacher/ headteacher to advise you on how they are keeping your son safe in class and play ground
I found the class teacher was VERY thankful for parents raising the issues with the headteacher!
Children at school learn very quickly who to avoid and how to stay away from tricky situations.
It's a life skill we all learn.
It's difficult when they are sat next to the child though, as one of my DDs was as she was good at not getting involved in the behaviours. I had to suggest to teacher it was now someone else's turn
Good luck and please be kind to the mother. I have a friend who was "that" mother and know how hard it is.