Bleach Dark Souls.
It suits a 6 year old because the only "blood" in the game is a move called "Blood Mist Shield" and thats only a title. It also has no offensive language. And the touch screen has buttons that are shortcuts to specials so he doesn't need to be a street fighter D-Pad pressing god.
However it doesn't suit a 6 year old because of one detail. He's 6? :\
What kind of 6 year old toddler is going to figure out the complications of making a combo thats more that 2 or 3 hits; the computer will slaugher him. Also Jump Ultimate starts is hardly for a child. Its in Japanese, and in order to progress in the game you have to blindly navigate through menus, missions, deck construction, and character creation. Sure the lack of language barrier makes this easy as pie for a teenager but for a 6 year old? I live around some 6 year olds, they're not that "smart" yet to tackle that kind of game. They're not even smart enough to stay away from the happyness candies and funtime vans.