These are all great recommendations, especially the Lego games and Minecraft. Some other games my 8-year-old boys have had fun with:
Cogs (tile-swapping puzzle game)
Madballs in Babo: Invasion (cartoonish action game)
Mini Ninjas (better for older kids I think, 10+)
Obulis (outstanding physics puzzle game)
Peggle (Peggle! nuff said)
Portal 1+2 (you already have this I hope)
Shatter (ultra-refined Breakout)
Toki Tori (platform puzzle game)
Trine 1+2
I’m amazed at how my 7 year old has 100’s of crafting combos memorized.
You should make your kid play SpaceChem. Tell them they can’t have dinner until they solve one puzzle. EVERY DAY. They’ll hate you, but they’ll grow up and cure cancer. So there’s that.
My 5 year old has liked watching me play Supreme Commander 2 since he was 3 and now loves to play it.
Star Wars Empire at War is also a good one. As long as you can give him AI teammates and make the opponents “easy” it is a lot of fun with exploding and stuff.