Block chat in Rocket League
Even if you change the config files manually, it can be undone from inside the game.
I think that simply telling your son to either not use the words he sees (let's be honest: he'll get exposed to it sooner or later) are not nice words or just disabling chat inside the game and telling him to not change the settings explaining your worries should suffice.
So, to block it from inside the game, open Options, go to Gameplay and set "Text Chat" (and "Voice Chat") as disabled.