OP, I supervise in school playground and there are certain pupils (thankfully very few) who are guilty of this behaviour.
It is particularly sophisticated form of bullying (the child directing the abuse appearing to have nothing to do with an incident "it wasnae me, I was over there") and I commend you for taking an objective view and wanting to do something to stop it. Regrettably, some parents will not believe us when we talk to them about their child's behaviour and, without parental support, it can be difficult to stop.
You are right to be concerned and want it to stop. I see pupils who have continued this awful, nasty behaviour (because they usually target vulnerable pupils to do their dirty work and watch from afar when they go on to get into trouble, giggling with their other friends in the background) through the school, developing it and embedding it into their character. They will deny, deny, deny having any involvement even when witnessed by staff. Their parents will always believe their story and support their child's position. It is frustrating because, as has already been mentioned in other posts, these children are not happy with their lives and we cannot effectively deal with this without their parents support and help.
I would suggest that you explain to your child that you will be working closely with the school and him, to help him to stop this behaviour. Don't talk about it in negative terms. Emphasise helping, supporting and encouraging the better behaviours you want from him. Ask for daily reports at the end of the day.
We sometimes use a sticker chart that a child will take home to show to parents each evening. A sticker will be earned for each period of acceptable behaviour. If your child's danger times are playtime, then a sticker for a good morning playtime, lunchtime playtime and afternoon playtime can be earned.
Praise every sticker earned. Talk about how he changed his behaviour to earn each sticker. Keep up the momentum. Don't give up.
He is still very young and hopefully, if you all (you, him, the school) you will have success, eventually.
OP, I wish you the best of luck.