To be honest, I would ask him what was up and why he was doing it. I don't like my ex generally, but he started doing this to a kid on my child's team after he learned that the kid was bullying my kid at practice (telling my child to shut up every time they spoke, that no one liked them, that they were ugly). It doesn't make it an acceptable response at all and my ex is immature generally, but I would still want to know the whole story.