Kirby games historically have rather gentle difficulty curves compared to their Mario and Metroid ilk, resulting in kid-friendly experiences without diluting the core game experiences. They always seem to incorporate interesting mechanics that are easy to perform and fun to watch. Kirby's Epic Yarn
would probably get my strongest recommendation, as the difficulty curve is gentle enough for almost anyone to play, and the art style is a sight to behold.
Don't discount some of the older Kirby titles on Virtual Console, either! The NES classic Kirby's Adventure
is one of my favorites, and also one of the best looking NES games out there. It uses a lot of contrast and bright coloring to give it a timeless visual appeal, and I think it's good looking enough that even kids today would find it playable and fun. It also uses NES-style controls, so to control it you simply hold the Wii Remote sideways, using 1 to swallow things up and 2 to jump (Virtual Console games also offer the manuals to read if you press the Home button, so check that out if you want to read how to play and then explain it to your son). Couldn't be simpler! And it's a rather easy game to breeze through, so for your son the difficulty would be appropriate.