NASCAR Kart Racing is a Mario Kart clone, but it's gotten decent reviews.
You might want to look into
ExciteBots: Trick Racing, it's coming out April 20. It's a little less Mario Kart since it's not a clone, but I'd bet any red-blooded boy would love a game like this, and it has better lasting value. I'm getting it myself
If you want to play it safe, go with NASCAR and he'll probably like it. It's not as good as Mario Kart, but it gets the job done. If you choose ExciteBots it will either go over his head or he'll love it. The better game is ExciteBots, but it may suit the age 7+ crowd more. If you think he can get into that, then he'll love it, but if you just want a decent, simple extension of Mario Kart, go with NASCAR.
I'd personally get him ExciteBots because A) I'm the kind of person who gets people things they may love instead of will like and B) I can't recommend a Mario Kart clone over a game that's better and more original when they're the same price ($40), but you know him better than we do, so your call.
Or you could get both.