If you believe that the coach had 3-4 players looking to hurt your nephew, then you could bring a lawsuit against him claiming battery because he intended to harm your nephew and your nephew was actually hurt. Additionally, if the board does not listen to your concerns, you could potentially bring a lawsuit against them for negligence (allowing such a coach to continue to coach children knowing that the harm that could potentially happen). This second lawsuit might be a longshot, but it would at least put pressure on them.
You could also just hire a local attorney to write a letter to the board saying that a lawsuit might be brought, but not actually bring the lawsuit. This could put pressure on the board by making it seem you are serious about the matter while at the same time avoiding the hassle of actually starting a lawsuit.
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