My son is also 11. We play:

Darkstone

Heroes of the Storm (just vs ai)

Minecraft

Empire Earth

Icewind Dale

Lovers in a Dangerous Spacetime

Minecraft Dungeons

Sonic All-Star Racing Transformed

Tomb Raider Temple of Osiris

And we build together sometimes too. He got RPG Maker MZ for Christmas. Amd he likes Scratch and showing off his projects.

I would say the ARMA games but they disappeared after the gamespy shutdown and came back but it doesn't say multiplayer or co-op anymore.

Can anyone confirm if it has LAN or some other way of using multiplayer if they have bought it after these games returned?

https://www.gog.com/game/arma_cold_war_assault
https://www.gog.com/game/arma_gold_edition

You've misunderstood: he actually keeps his children in a chicken coop.

My son and I love playing Spintires and Mudrunner together in co-op over Steam. No violence or bad stuff, just hauling some logs through the Siberian forest. Starbound also has co-op.

If they're into shooters, why not Plants vs Zombies Battle for Neighborville?

Hmm if it was more than 2 I'd go with some type of brawl multi-player game. But just two, if sharing a screen probably split screen of some description, racing, or a puzzle game, if you want something more fighty Sacred 2 can be played coop (though i don't remember if split-screen was just the console ports)

Taking turns, you could do something like Beat Buddy or Rayman legends. (Good games).

Lovers in a Dangerous Spacetime, this is a game that HAS to be played with 2 players. (Though single player you do get an AI dog where you tell him which spot to take and he'll manage the ship) Basically there's 5 stations and each player can only man 1 at a time, so you can move, shoot, shield, special attack, and ummm.... other things i forget.

I'd suggest maybe a DDR thing if you had a pad and some good music.

Assault android Cactus, play different quirky robots against machine bugs!

Not Octodad... while multiplayer it's more annoying than fun.

Pixeljunk Shooter is good

Ringrunner (A ship allows coop where you can assign weapon groups to player two)

No, YOU MISUNDERSTOOD!

What about the Borderlands series? I finished the first one in splitscreen with my flatmate years ago. Could see how a kid that like Fortnite could get into this?