You answered your own question, it's the coach's call. He is being asked by the coach to help his team by pitching. If other players take issue with your son, that's on them and shouldn't be his concern. If coach says you need to pinch hit, you grab a bat. If he says pinch run you grab a helmet. If he says go close the game you head to the mound and give it hell. These young men have all been playing this game since they were kids, they should all understand this well by now.