These kinds of issues are endlessly frustrating because it's quite clear that playing sport shortly after a period of illness is not in the child's best interests, but unless you have a certificate from the doctor stating that he is not fit to play sports yet, then, unfortunately, there is not a great deal you can do here.
Basically, it is likely a that the court will not consider this a major issue in the greater scheme of things because a) sporting issues don't detract from a child's right to know and be cared for by each of his/her parents; and b) realistically, involvement in sports is a positive thing for kids and it may go against you if you choose to stop him from going without reasons validated by a medical expert.
I am sorry if this isn't what you wanted to hear - children's matters are frustrating to no end when your views about parenting are so vastly different from your ex's. Just keep an eye on the matter and if he begins to show illness again, take him to a doctor and request certification that he be excused from playing sport for a while.
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Hope this helps.