Steam Game for 6 years old Boy

Bad rats and nyan cat game.

No seriously, the car football game called Rocket League. Maybe some of the Trine games too.

Draw a Stickman: Epic

Leave him Outside, he needs to gather some xp
Jk :P, educational games are fine, humongous games, maybe, there is also a remake of zoombinis on steam that teaches logic
Of Course, simple platformers and racing games are great aswell

Oh, Minecraft. It's not on steam, but apparently every little kid loves it.

"The Elder Scrolls V: Skyrim - Legendary Edition" of course with mods :)

And the game which You own:
Mark of the Ninja.

Have a nice week!

Anything form Humongous Entertainment would certainly be age-appropriate.

The Counting Kingdom

little inferno does a great job too and is not too hard to play :D

Maybe take a look at "Crazy plant shop" : http://store.steampowered.com/app/292200/
Cute and funny, it's an educational game about genetics...
Maybe a bit too much for a 6 yo, but he may find it fun to breed plants and sell them anyway and it's totally violence free !

A game I like a lot is "Craft the world" : http://store.steampowered.com/app/248390/
Not sure if it's suited for a 6 yo... Check it you may know better than me :)

Another game that may be suited for a 6 yo is "Letter Quest" : http://store.steampowered.com/app/373970/
English isn't my mother tongue so it was pretty fun, educational and challenging even at 30+ yo :D
I'm sure someone who is learning to write and read will find fun in it ! Plus : the game is totally cute !

Botanicula
Crazy Machines
Doc Clock: The Toasted Sandwich of Time
FEZ (although this may be hard, depends on a kid)
Machinarium
Quantum Conundrum
Scribblenauts
Tiny & Big
Trine
World of Goo
Zero Gear

a story about my uncle , any game from puppygames (titan attacks,ultratron etc.) , sports games , type:rider . Even tho pokèmon is the perfect game for children , it's designed for children and should be played mostly by children.

Freedom Planet
Rayman Origins/Legends
Castle of Illusion
DuckTales Remastered
Lego Games
Max: The Curse of Brotherhood
Mega Man Legacy Collection
Scribblenauts
Sonic Games

Chroma Squad!

How about PixelJunk Shooter? It's fun if you have two controllers, you can play together =)

I don't really know which games fit 6-year olds, though O_o

Edit:

Maybe something like PARTICLE MACE, Bridge constructor, or Tiny Bridge: Ratventure?

Also, this isn't really a game, but maybe it would entertain him and teach him stuff too: The Powder Toy =)

Tribloos 2
Putt-Putt games

My son loved Plants vs Zombies at that age. http://store.steampowered.com/app/3590/
He also enjoyed Scribblenauts a bit (but not nearly as much as PvZ): http://store.steampowered.com/app/218680/

EDIT: He currently enjoys Minecraft Story Mode (he's 8 now). http://store.steampowered.com/app/376870/

FNaF xD

A Lovely FPS: Lovely Planet
http://store.steampowered.com/app/298600/

Toki Tori 1 & 2, Sigils of Elohim, Race the Sun, LYNE, Goat Simulator, Tengami, Circuits, The Bridge, Type:Rider...

He'll always be asking for GTA V no matter what

I would say the Sega Genesis collection is a safe bet http://store.steampowered.com/sub/7825/ as well as many pinball games.

Any LEGO game.

Teddy Floppy Ear

Emulator + whatever you've played at that age.
For me it was something like this: Mario (Wario, Yoshi, Donkey Kong), Super Ghouls 'n Ghosts, Spyro, Crash Bandicoot, Oddworld, Final Fantasy, Street Fighter, Tekken, racing games....