P1 and bullying

[Babybrain26]

Our son just started P1 and was getting on great up until about 3 weeks ago. We noticed a change in his mood at home and eventually he told us that a friend started sticking his tongue out at him and told him he could no longer play with their small group of friends. Our son told the kid not to stick his tongue out at him, but apart from that I'm not sure what could've maybe caused the isolation. Another child stepped in and said he would play with my son when this other kid wouldnt play with his usual friends. We messaged the teacher and they caught the child in the act in the playground so they spoke to them individually and to everyone in the class. It seemed OK for a week or so but I've noticed that the 'bully' has been trying to befriend the child that's stood by my son and wanted to play with him (isolation?) I think he's been telling him not to follow them round the playground and last week the 'bully' stole items from his desk and stuck his tongue our at him again in class. The teacher put him out of the class until he apologised. How should we deal with this? It is upsetting him. I don't want it to have a lasting effect on him, what do we teach him to say to this kid? Or how do we advise him best? Our son is 4 the other kid is 5.

It sounds like the school is doing pretty well so far.
A 5yo shouldn't be labelled as a bully. Don't use that word as a noun. Bullying behaviour should be described as transitory and temporary not inevitable. He's a kid who hasn't learned how to be a good friend yet. Your role is to do whatever you can to boost your child's confidence and understanding so that he doesn't take these incidents to heart. The school's role is to teach all the children better skills but that's not going to be instantaneous. Meanwhile obviously they need to protect your ds from any harm.

This is a very sensible answer.
I’m not comfortable with such a young child being labeled as a bully neither and I am pretty no nonsense when it comes to behavior.

I'm not sure how it's all came about if I'm honest, at the start this child was literally hugging my son, now it's the complete opposite. Also the playground supervisors are trying to keep an eye out for it but of course it's still happening. What should I say to my child? How should he react when he's being excluded? Told not to walk near his friends etc. This child doesn't act like this with any other child.

Tell him to laugh and say "don't be silly"
Tell him he can play with anyone and walk with anyone. Tell him he can be confident to not be controlled by the other kid.
You can try telling him the other kid is confused and probably scared and trying to control stuff he isn't supposed to be in control of because he hasn't worked out how to cope with something. He feels weak and is looking fir someone who feels even weaker so he can pretend to be strong, so being strong (strong as in determined and confident, not physically strong) is the antidote. However, your child doesn't need to understand why the other kid is being unreasonable if it's complicated.