We have a general rule when it comes to media in our house: we check it out first and then make our decision. No one gets to play or watch anything without us checking it first. And then, it has to be played in a common-use area; no tvs or consoles in bedrooms. When they reach a certain age, the rules relax...but if they give me cause, the rules get reinstated.I know. Just let him play what he wants. At least he's not bugging you to buy another $60 game. It's... free. However, he'll grow up and realize how stupid Fortnite was and feel shameful for loving it, playing it.
I encourage you to recommend some games to him to show what he's been missing. Just say something like, "ya know... there's Mortal Kombat, y'know."
You seem like a cool parent though. You seem to want your kid to experience the best of the game industry, even if he's exposed to something like Mortal Kombat. Lots of parents go nuts and take away Mortal Kombat from their kid, and shield him/her from that kind of content.
One of my kids is special needs so we have to be exceptionally careful with what he sees/plays. He struggles distinguishing fiction from reality and there's a real risk to his safety if he tries to act out what he sees. He wouldn't know it wasn't safe and wouldn't understand why either.