The Cars characters are at MGM, and they are in the parade there as well as make appearances. They can't really sign autographs since they are, um, cars, but one suggestion I read is to have your son place an autograph page on the tires and run his fingers over it and then he'll have a sort of dirt mark of the tire. If you place a protective sheet over it, it should stay.
I second the Nascar Cafe at Citywalk. You can take a cab there or a Mears bus if you don't have a car. Citywalk is at Universal Studios (kind of seperates US from Islands of Adventure). But they have a lot of nascar stuff at the cafe from what I see in the pics. Some of the cars are out front (Jeff Gordon's, Rusty Wallace's (now Kurt Busch's), and some others. And there's a lot inside, too. And all the food on the menu has a nascar theme. And I'm sure they have televisions showing races, etc. There's probably even a gift shop or something. (I have never been there, just speculating). FI and I are huge nascar fans and plan to eat here on our disneymoon. It looks like a lot of fun.
I also think it would be cool to take him to Richard Petty's, even if it is just to walk around, watch others drive/ride, go to the gift shop, etc.