I recommend UR MOM!!!! JK, I recommend Dota 2, Battlefield 4, Farming Simulator (he needs to build some character), Bayonetta and Playerknown Battleground!
How about Rimworld? have him manage a settlement on a new planet, creating a community of colonizer from scratch, and hopefully, doesn't make them all starve to death or turn to a cannibalistic cult.
a ps2 emulator would give you access to a great catalog of games kingdom hearts definitely spyro rayman jak and daxter ratchet and clank tak and the power of juju prince of persia the sands of time etc
Well some already recommended pokemon. Pokemon red to emerald use vba emulator Platinum to b2w2 can use desmume You can also try Warcraft mainly Dota 1 DragonNest would be good Ragnarok either offline or international server League of legends (just teach him to mute chat) Elsword if its available(semi 2d Mmorpg) Flyff
Kanojo X Kanojo X Kanojo. It's a R-10(+8) eroge hahaha... Kidding try RAGNAROK and don't forget, never hunt the porings or the porings will kill you FINALS FANTASY series
Stardew valley (a cheap great harvest moon clone) Starcraft Diablo 1-3 Warcraft 1-3 Black and white 1-2 Doom (old one) Duke Nukem 3d Quake Monkey island Grim Fandango Day of a tentacle Indiana jones: secret of atlantis Need for speed All Lego Games Overwatch Heroes of the Storm Heroes of might and magic Trine 1-2 Magicka Age of empire 1-2 Command and conquer red alert Emulator games: Snes: super mario 1-3 + World + cart, yoshis island, donkey kong country, zelda, gooftroop, loony toons and Turtles in Time. Sega: sonic, earthworm jim, tas the tasmanian devil going to outer space and Duck Rogers. Ps1: Crash Bandicot, Jak and daxter, Oddworld, FF 7-9 Ps2: Jak 2-3, Ratchet and Clank, FF X, Kingdom hearts. I Hope this is enough for a while xD
my little brother when he was 7 is playing red alert 3, Starcraft 2, left 4 dead, San Andreas, plants vs zombies and some MMO
Give him Harvest Moon/Rune Factory games. After that you can introduce galge/dating sim games. Then slowly, pull him into the world of Eroge. Corrupt that little brother of yours my friend... make him one of us...