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.
Obviously you haven't played Extra Mode in Kirby's Return to Dreamland. It gets unlocked when the main mode is cleared, and the difficult spikes greatly. Not that the main mode is always a walk in the park, either.
Kirby games are designed to be playable by all, and challenging to expert players. Even Kirby's Epic Yarn, which has no "game overs", is difficult to master.