Son being pushed

Hello All!

It has been a long time since I have logged in to the bump and my first time posting to this board but I am hoping that some of you will be able to provide a few suggestions. My son is 20 months old and is very laid back. Recently his 23 month old cousin has started playing very rough. I contribute part of it to the fact that the cousin's teenage uncle plays rough with him but I get extremely frustrated when my nephew continuously runs up behind my son and pushes him to the ground for no reason. Earlier this week he was pushed into the corner of a kitchen cabinet. That time my nephew's father actually jumped up to discipline him but it didn't last long. My MIL picked my son up from the babysitter a couple days ago and watched him for a few hours and when she brought him home she told me that he fell into a toy at the babysitter's house and his lip was a little puffy. I couldn't help but wonder if he really fell or if he had been pushed again because the babysitter watches my nephew occasionally too. Has anyone else experienced anything like this. I don't want to keep my son away from his cousin because they do enjoy playing together. However, I don't want to wait for my son to really get hurt before something is done about it.