It's difficult for kids to wait. I understand he wants it for Christmas. It is an important day for kids and families. As a father, I always understand what I wanted when I was that age whether it was five or eighteen. Based on that, I would buy it and be my kid's hero on Christmas. The month would be a long wait, and waiting until after Christmas would probably make no difference in the value of the iMac to your kid.
Now, for some people Christmas and being a hero to kids means nothing... Those are arguably the smartest ones of all of us. They are the ones that live unhappy lives and would rather work than being with their family. Hey, I will take being called stupid to make my kid's Christmas anyways.
As long as your kid appreciates you, and is generally a good kid, buy the iMac for Christmas.