Torchlight is great. My son loves that game ( he's 5 ). He also plays Warhammer 40K Dawn of War with the blood turned off on easy difficulty.

Otherwise he likes to goof around on those flash games websites and play stuff on there, Crush the Castle and Sticky Ninja come to mind.

The only ones that I can think of are
Hearts of Iron 3
Europa Universalis 3
Crusader Kings 2
Sim City 4
Tropico
Though they might be a little complicated. Hearts of Iron and Crusader Kings being the most complicated. EU3, Tropico or Sim City being the least. Sim City and Tropico perhaps being the most fun for a youngster.

Torchlight is good gameplay wise but you're going to have to help with skills and stat points. It isn't that complex but probably beyond a 6 y/o.

I sit with him while playing Torchlight as a guide. It is fun, just point and click at the evil monsters/spiders/bad creatures and your character does all the hacking. A good feature I like is that one can check off blood as an option so it is not too gory.

Plants and Zombies have been a hit as well. Even my three year old daughter enjoys that, and I had to get it on my iPhone for her as she cannot handle a PC mouse yet but is an expert with the iPhone interface.