Super mario galaxy- great graphics, amazing gameplay, BUT, they are coming out with a second one soon, so depending on when your bro's birthday is, you may wanna wait for that.
Call of duty modern warfare: reflex edition- this is likely your best bet if you have internet access for wii. If not, well campaign is fun, but kinda short, you can replay the missions again. But the online is the best part about it. But eiyher way, a great buy. Best shooter on the wii. Amazing graphics, great gameplay, and awesome online action.
Mario kart wii- this game is great. If you have online or not. Single player is highly addicting. The online mode is good. There are tornaments in which you do different things and try to beat other people around the world. But mainly, you race other people around the world.
These are my top 3 games, if you have online, DEFINETELY, NO DOUBT, go with call of duty modern warfare reflex edition. But if not, super mario galaxy will keep you entertained for months. (That is one thing i forgot to mention, the main mode for super mario galaxy is REALLY long, and it is all fun.)