That's a tough question, 2D action platformers are practically dead. There are none aside from Donkey Kong Country Returns, Super Mario Wii and Kirby's Return to Dreamland, but there are games that look similar, such as Kirby's Epic Yarn. It would only be fair to warn you that those games just look similar, but they play like collectathons, the difficulty is insultingly low and it's all about just collecting stuff. Maybe your son will like those games, since everything is new to children, or maybe he will just get bored because there is no challenge in them.

There is the Super Mario All Stars collection, it contains the first three Super Mario Bros. games on one disc. It's basically the old Super Mario All Stars from the Super Nintendo wrapped up in an emulator, but it's still tons of fun. Especially the first Mario is a great game for players with less skill, it was after all the very first Super Mario game. It was only released in limited quantity, so getting a hold of a copy might be quite hard.

I wouldn't recommend buying WiiWare or VirtualConsole games because Nintendo has no account system. Once the Wii breaks your digital games are gone (you can redownload then any time but only on the console you bought them on). This is worse than any DRM ever. Plus you don't even get manuals, which is a deal breaker for games like Zelda on NES where the manual had the world map. Considering that the games are way overpriced for something you don't ever own.

Another idea is to hack your Wii, then you could run an emulator for old systems such as NES or SNES, they have very fun games like the Disney games from Capcom (Duck Tales, Resuce Rangers) which will never see a re-release for licensing reasons, so there is no harm done in pirating those. I have hacked my Wii myself and I play all my (legally bought) retail games off a hard drive; no more disc drive noise, no switching discs and I can even use cheats, like increasing running speed in Xenoblade or having all the big coins in Mario from the start.