My suggestion is to stop being an overprotective parent (I mean this as honest advice, not to be mean). Bottom line is that you can't just stop this type of behavior, and your son will be exposed to it regardless of what game he plays. When (or if he is already in one) he enters high school, you won't be able to protect him. There are mute + censorship option in the client for these purposes, as well as the fact that Riot bans/chat-restricts players regularly.
Now I'm not trying to justify these player's behavior though. I've never been the player you describe, but I'll freely admit that people in general (it's definitely not limited to just League players, this applies to everyone, especially on the internet where being anonymous is a common shield for bullies) are harsh and cruel. You can't shelter your son forever, no matter how much it sucks.
There's also the fact that imposing force to stop your son from playing this game because your concerned for his feelings will only create a rift between you two. Trust me, you don't want to be that parent that just forces your children to do things "because you say so". This is one of those things that you need to let your son decide for himself, else you create a very controlling and restrictive environment for your children.
But hey, I can't tell you how to parent your child. I simply am trying to express my point of view as I can relate to both you and your son.
My advice: Try to consider his feelings instead of trying to protect him from things that he'll be exposed to anyways
