Hi Leah,
You son sounds like such a great kid. I love his interest in animation!
Unfortunately there are no "old time" characters available to greet in the parks. On the plus side, there is often Oswald and golden era Mickey merchandise for sale in the parks. The mix may change by the time you get there in September, but I know of a great Oswald tee being sold now. You can see a photo of it on the independent website thedisneyblog.com in an article dated 4/12/12 called "Disney World Merchandise Round Up." There are also often books sold in the parks that give the history of Disney animation.
You should be sure to visit Disney's Hollywood Studios. The "
Walt: One Man's Dream" and the "
Magic of Disney Animation" attraction are both up your son's alley.
And on an only somewhat related note, if you ever find yourself in San Francisco, you should be sure to spend an afternoon at the
Walt Disney Family Museum. This truly is the epicenter of information about the development of Disney animation.
I hope you and your son have a wonderful vacation.