Hi Katy!
My two boys love Buzz too! You won't have to go far to find a great selection of Buzz Lightyear and Toy Story Mania merchandise, as there is a "rocket-shaped" souvenir cart right outside the Buzz Lightyear Ranger Spin Ride. In addition, as you exit the ride, you'll actually walk through a small gift shop filled with "Buzz stuff." The Emporium on Main Street U.S.A. (the large gift shop on the left hand side as you approach Main Street from the park's entrance) is the largest store in the park, and also carries Toy Story/Buzz Lightyear goodies.
Since your little superhero is such a fanatic, I have to share that I recently picked up two navy blue Buzz Lightyear track suits for my kids at a local New Jersey Costco. They practically live in them! I have seen them several times over the past few years, and have seen them on eBay as well.
I hope you have a wonderful vacation!
P.S. If your son is unable to wait in line for prolonged periods and/or would have an easier time waiting in quieter areas, please visit Guest Relations at the Magic Kingdom (it's located in City Hall) for a Guest Assistance Card (GAC) as you enter the park. It will be good for the duration of your visit, and should facilitate an even more magical trip! Please click
HERE to read one of my earlier posts about GACs.