I'm also a one-handed gamer (Cerebral Palsy) and I gave up on the Wii because of the lack of quality games for people in my situation.
If it isn't too late I would suggest your parents take the Wii back and get him an XBox. I believe the 4 GB is now the same price as the Wii ~ $200. A majority of the xbox games I have played have options to customize the button layout however you see fit. And actually Just the other day I saw ALL the sonic games were on sale for 50% off, with the most expensive one being Sonic 4 @ $10.
I have had no problems adjusting the controls of many popular games games like FIFA, Madden, Tiger Woods, GTA4, Red Dead Redemption, MLB2k10, and Need for Speed to make them easier to play one-handed.
The only games I have come across so far that I have found it difficult to play one-handed are Modern Warfare 2 and the NHL 11 Demo and to be honest I have not given either of those much of a chance and they might be playable as well with some practice.
If it turns out you have to keep the Wii, you can also get one of these
which plugs into the wimote and can be used with many of the classic mario or sonic games, but I found rthe response time to be pretty sluggish which actually made Mario brothers even more frustrating for me than using the regular wiimote.