Ok I've had a look ate their website, it looks like to play a Wii game on a Wii U you need a Wii controller not a Wii U one (does your grandson have a Wii as well?). Maybe ask in a store like Game for proper advce. This is pasted:
Enjoy nearly all of your existing Wii games on your new Wii U console
Wii Menu is built in to Wii U. It allows for backwards compatibility with Wii game discs, saved game data, and purchased WiiWare and Virtual Console games on your new Wii U console.
Wii games cannot be controlled with the Wii U GamePad. Use your existing Wii controllers to play.
Wii games will not display in HD on the Wii U console.