Thankfully? Why don't you just forbid him to play Fortnite? Sounds like your son doesn't listen to you, and that's a problem.
Schools group together only one age of kid for socialization and only 20-30 of them. If your kid is not into the same thing as enough of the other kids in that group, they will likely be ostracized, even unintentionally. So you must let your kid do the things their friends do.
Broader society does not restrict the age of who you can socialize with. My friends vary in age quite a bit. My friend's kid can play with my kid despite being a different age, but that's much less than the 30 hours a week spent in school.
This kind of approach is also invalid. So what everyone plays Fortnite? There are many places to get socialized with other kids. The kid likes basketball? Sign him up to a basketball team; he likes to play music? Sign him to some band; etc. Kids shouldn't surrender to peer pressure.
I agree schools are also a problem, but not the main problem.
If so, was it a problem that you didn't listen?
I'm not a parent, but fortnite is not like smoking or drugs, common. If you don't let kids grow over this kind of bad content, they will never become good discriminators.
What show did you like when you were a kid? Do you still like it? Are you eager to consume it in the same amount as you did? If not, it means you grew over it.
When I was a child I loved the Monkey Island series, completing them several times a year. Solving the puzzles made feel smart, and the jokes made me chuckle. But now? I could hardly complete the high-res remakes or even the latest title. The puzzles are either too simple or just nonsensical try-and-error, I find the story boring and shallow (compared to other content I consume now), and none of the jokes really hit the spot anymore.
South Park even has an episode about it: https://www.youtube.com/watch?v=zxRFQ_333_Y
I think forbidding kids to access content they crave is wrong and cruel, as you are basically forbidding them from exploring their tastes, forbidding them from becoming interesting adults with refined and deep tastes.
Even in the case of alcohol, I think people should explore it and their limits (getting wasted) when they are young. I have an aunt who never tried alcohol until she was in her 40s, and she went through the same phases as teenagers; however, at that age getting too drunk too often has serious social consequences. She spiraled out of control, becoming alcoholic, and then later addicted to other drugs. I am not a psychology expert, but I always thought that her problem was that she was too old to explore this path.
In my country we have an idiom I like: "potro que no galopa, de caballo se desboca", which means "colts that don't gallop become wild horses".