Hi Lizzie,
I always liked the Wizard of Oz too. That is interesting the way your son reacted to the chair. Did he hear someone talking about it while you guys were there (even one of you) and he was just repeating what was said by chance? Or was there a small sign there with a brief description and he was able to read a few words and added to it? Was this chair used by a male member of Judy Garlands family?
If he didn't hear someone talking about the chair and it did belong to a male, he very well may have picked up an impression from it. Objects can hold energy. His actions do make it seem like that is what he was doing actually. Has he ever done anything like that before, or anything else happen that you would call unusual? Does your older son still do it, or have anything different happen?
We all have innate abilities and for whatever reason for some it's more notable. I think when children are discouraged from letting what comes natural to them be, it can often start to close that door (so to speak), however with some it will still occur and even increase in their teen years (did for me) or at some point as an adult. =)
How do you feel about it? Do you think that is what he was doing, and are you ok with it if it was? How was he afterwards, did he talk about it more, was he bothered by it? Is anyone in your family sensitive, or psychic, an empath, or a natural medium? It can skip generations btw. If he was not bothered by it then I think just wait and see if he does it again, or if anything else happens. I believe we all(children included) will have times of what I term as a psi moment, as adults we may brush it off, call it a concidence or imagination, or wonder how did I know that, or why did I feel that way, or we hear or see something that we can't explain. He may have just had a psi moment, time will tell if it's something more (if he is a sensitive etc), for both or your sons actually. Keep us posted. =)