Kelly,
If I could only go to 1 character meal, it would be
Chef Mickey's at the Contemporary Resort. The setting is great, with the monorail running overhead a really cool sight. I also find the food to be pretty good, and the character interaction (Mickey and friends) to be great.
As for Lighting McQueen, the best place to meet him (yes, you can, in fact, meet him!), is at Hollywood Studios. You will find that character meeting locations in that park can change occasionally. So what you want to do is ask a cast member when and where your son can meet him and Mater. They have a central line where they can look this up. You might even consider asking a cast member the day before you are going to be at Hollywood Studios, even if you are in a different park. That way if there is an early morning opportunity you can take advantage of it.
FYI, I think that the McQueen/Mater character greeting experience is one of the best in all of Walt Disney World!
Send me more questions and I would be glad to help out further!
Have a magical (and delicious) vacation!