I’d address the behaviour clearly and calmly.
He does not need to stop enjoying his team’s success, but celebrating an opponent’s mistake is different from celebrating your own goal or tackle.
I would say something simple away from the heat of the moment: “We celebrate what we do well. We do not celebrate somebody else getting something wrong.”
Then give him a replacement behaviour. If the other player makes a mistake and your team wins it, celebrate the teammate who reacted quickly rather than the mistake itself.
I would not label him disrespectful or make him apologise publicly unless there is a specific incident that needs it.
At eight, competition and empathy are still developing together.
You are not trying to make him less competitive. You are helping him understand that you can really want to win and still respect the people you are playing against.
