Steam Game for 6 years old Boy

Check out PopCap games, they have a lot of fun casual games. You could also try World of Goo maybe.

Has anyone mentioned garry's mod?
If game is a must I would recommend lego series.
But I would really like to recommend anything but any video game. They can become quite addicting both for younger and older minds alike.

DOTA 2

Someone suggested CS GO, I can only add any Call of Duty game.

Toki Tori is by war a better option as it will work as a brain stimulant as well.
Toybox Turbo is fun as well, as boys and cars can never go wrong, put some hammer and guns into it and its a win win.

Many repeats, but here's the favorites of my 6yo daughter:

Sonic All Stars Racing
Castle Crashers
Mount Your Friends (she thinks they're all wearing poopy diapers!)
BattleBlock Theater
Goat Simulator
Turbo Dismount
LEGO Star Wars

I just picked up the Teddy Floppy Ear games for a buck, I'm sure she will like them as she loves point & click adventure games - we just played through the Star Trek 25th Anniversary game (not Steam, GOG has it) together and had a blast! (She likes Star Trek)

Well, I suppose he'll like Lego games, but as a scientist, I think it's much more important for him to play an educational game, like Professor Why™ Chemistry 1 (http://store.steampowered.com/app/372950/) and similar games. ;P Education + gaming = fun! :D

Postal 2.

and seriously try HOG games, they are just for around this years I guess, best I played till now was Grimm Legends 1 and 2 :)
well... my cousine loved it and I loved it to :P

just requires some logic thinking and creativity not too hard, I know kids love it, and hidden objects part are the best, while any harder things can be skipped or you can use "tips", it also uses some nice stories from around our age like of Andersen or Grim Brothers

http://store.steampowered.com/app/279800/

Child of Light, Rayman.

dead space, super hexagon, aoe, poker night. bad rat, castle crasher, racing game, draw a stick man epic , limbo robocraft, simulator game, never to youg to learn mechanic, Hercule games on steam, dmc, resident evil, and my favorite when i had that age, unreal tournament, pretty fun

Toybox Turbos looks like a fun game for all ages.

This one's a cute little point and click adventure: Witch's Pranks: Frog's Fortune

My son is 6 years old and likes the following games:

8bitboy
Evoland
Fez
Jones On Fire
LEGO Harry Potter: Years 1-4 - Excellent local co-op if you want to play with them

I'll also be watching this thread with interest as I was going to post something similar myself as I'm building him a PC for xMAs and was going to ask for game suggestions too :)

Honestly, i strongly reccomend about Besiege, ( and Trove ?) then all Lego series, Sonic Series are optional.
Other than Steam, how about Minecraft? It's good for kid tho ^^

Disney games...or Lego

They have a family friendly section of games on steam. http://store.steampowered.com/tag/en/Family%20Friendly/#p=0&tab=NewReleases

some that i see right now:
Plastiland
Wildlife Camp
Disney Infinity
Minecraft

Here's educational games too
http://store.steampowered.com/tag/en/Education#p=0&tab=NewReleases
Zoombinis
Doodle God

There's also the casual games like Floppy Ear and Putt Putt
Floppy Ear Bundle is on sale right now for 90% off http://store.steampowered.com/app/371420/

Scribblenauts, Sonic & SEGA All-stars Racing. Sonic & All-stars Racing Transformed, Race The Sun, NiGHTS: Into Dreams; Sonic Adventure DX, Sonic Adventure 2 Battle, Guacamelee: STCE and Freedom Planet.

Those are my recommendations for a 6 year old. Most are SEGA titles, but I also included a brawler and good Sonic clone game.

Call of Duty

All those lego batman games, minecraft, the sims, rayman, worms reloaded..

Plants vs. Zombies GOTY Edition

fairy solitaire
no idea if it has been suggested before, anyway my kids loved it around 6.

As an FYI Nic the Teddy Floppy Ear - Mountain Adventure is currently 90% off on steam. (D/K if someone already pointed that out)

The deer or Pat & Mat or Peggle

Chess