My son compares himself with the same teammate every week

I would try to move the comparison from outcome to personal progress. Telling him “do not compare yourself” probably will not work because he already cares about this teammate. Instead, when he says “he scored more”, you can ask what he did today that he could not do a few months ago. Keep the teammate out of your judgement. Avoid responding with “yes, but you defended better than him” because that still keeps the other child as the reference point. You can also praise things that are specific to your son’s development rather than rankable outcomes. Friendly rivalry is not automatically unhealthy. The concern is when his mood depends entirely on beating one person. Help him build more than one way of deciding whether he had a useful day.