@Neolara
I would talk to the school. It's not verging on bullying. It is bullying. Sorry your son is going through this.
This ^^.
It is bullying.
I have three kids, and middle one had this situation, always tricky when their parent is part of wider friendship circle.
I went through school. Always go through school. It happens at school, Oliver’s mum can’t police what happens at school, and in my experience, most parents get very defensive in this situation.
Let school handle it. Contact them by email (I would email form tutor and also head of pastoral affairs, who is usually someone a bit higher up the hierarchy); state how many times it has happened; explain how it makes son feel; say you look forward to their response of how they will implement their anti bullying policy to make sure this will no longer be happening, you are so grateful for their support in this matter… and if you don’t hear anything by end of the week, forward it again, moving up the SLT hierarchy - maybe to an assistant or deputy head this time.
I would leave it up to Oliver’s mum if she contacts you about it.
I have been you with Dc1 and DC2. In dc1ma case, when I turned up at primary next day, Oliver’s dad burst into tears and cried about how mortified he was.
In DC2’s case, her friend’s mum responded with a ‘I’m sorry your DC2 and you felt she had to complain to school about this’ apology. Our friendship has not recovered, and I am civil at group gatherings but make sure I am the other side of the room for most of the evening…
It’s great your son is telling you about this, and he is very lucky to have you fighting his corner with him.