Amy,
I saw
WALL-E last night, and also thought this little robot was someone I'd love to meet in person. Unfortunately there aren't any scheduled meet 'n' greets at the WDW parks right now. To find out more info, I contacted Disney and was told that "WALL-E is still out cleaning up the universe and isn't currently available for meet 'n' greets." ;-)
Considering how popular and successful the movie is (some critics are saying it is the best Pixar movie to date!), I imagine it won't be long before he is given a break from clean-up duty to come out and meet his fans. No guarantees, but I certainly hope I'm right.
I highly recommend you ask a Cast Member when you are at WDW to find out if he might be appearing anywhere in the parks -- usually they can call the "Character Hotline" to find out the latest news on meet 'n' greet opportunities. You can also always find helpful meet 'n' greet info in the Times Guide at each of the parks.
One other source I sometimes use to figure out character locations is www.wdwautographs.com. This unofficial WDW fan site details where to find characters and how frequent/rare they are in parks, as well as show examples of their autographs. Unfortunately they also don't have any information about WALL-E yet.
I certainly wouldn't promise him anything, but I hope your son gets a chance to meet WALL-E in September!