I heavily recommend downloading some older classics from the Eshop if you get the chance. It will really open his eyes to the amount of different games there are and overall really help his gaming knowledge. Try stuff like Super Mario 64. When he gets older show him games like Pokemon Mystery Dungeon Explorers of Sky if you don't already have a 3DS. That is the one thing I really enjoy about the Wii U is that there are lots of classics you can download for a fraction of the price they used to cost. I hope your son enjoys it!

Your son looks traumertized in that picture! Like the gamepad is holding could attack him at any moment!

Super Mario 3d World would be a great pickup and one you could play with your son if you wish.

Donkey Kong Country is a good game but I believe they are quite difficult.

Mario Kart 8 goes without saying! The best Mario Kart game to date.

Windwaker might be a good bet, unlike twilight princess it's more kid friendly.

I think the more obvious candidates have already been covered so I'll recommend you the first game I bought and played for WiiU.
Rayman Legends. It's gorgeous, cheap to buy and super fun. You can play through the entire game in co-op and the musical levels are amazing. Definitely something I'd recommend to you and your son.

DK tropical freeze is near flawless as a game, and it also has local co-op, but it can be tough as nails in places, so it might be alittle too hard for your son.

Also Captain Toad Treasure Tracker and Yoshis Woolly world are super charming and neither too challenging nor boring and both have a lot of exploration based content.

Be sure to check out the eshop as well. There's a lot of VC games you'd probably enjoy. I know I loved the 2D Zeldas as a kid.

Rayman legends and mk8. My story is identical to yours but my kid is now 5. Also for you, black ops 2 with dual screen multiplayer. No screen cheating! 

Hated red the thought of the game pad and then Loved it after a day or two. 

My nephew loves captain toad. My niece goes nuts for SM3DW. Well both do now. He mains Toad and she mains Luigi. Otherwise I recommend not forgetting Wii mode. They love the OG Wii Sports and this carnival game I found cheap. Otherwise since you got Splatoon on the way I think you're off to a great start!

I'd say you'd love them so. I'm guessing by your attitude, if it doesn't have sex and blood it's only for kids, you are more than likely a child. 

Why does he look so scared in that picture?

Wow, you saw an oportunity to say Nintendo games are made for 4 year old kids without consequences and you couldn't let it escape, don't you?

That's like saying Ghibli or Pixar makes films for 4 years old kids just becuase 4 years olds kids can see them too. 

Chill out, don't take it so personally. I just said Nintendo games are usually aimed towards kids. Do you think Nintendo make Mario to carter to Adults? I buy tons of Nintendo games but I know they probably aren't targetted for me, doesn't stop me from buying them :)

He looks really scared in that picture, like he's been threatened to smile, haha.

I know he's probably just tense because of the camera but it's funny.