Super Mario Galaxy is an excellent game that features a unique 2nd player "co-star" mode that let's another person play along side with Mario. It's great if one player is good at games and the other player typically watches. Wii Play is also not bad if you are thinking of getting an extra Wii Remote. If you're asking about non video games, nothing beats Legos! :)

It's abit late but i'd reccomend some good ole fashion Legos. It's a great activity to complete a 1000 piece lego together, you'd get a chance to teach him some basic reasoning and logical deduction skills!