I would probably wait, depending how soon your going to run it after Christmas and physically fit him. A proper fitting seat would be more important to me than if it had a seat when I give it to him/her. You want your driver SAFE and comfortable, espesially with a jr driver. JMHO
If you're buying it from a local source, maybe they'd let you trade it in for the right size if it doesn't fit. I learned with my kids that "here's a picture of what you're getting" doesn't get them all that excited.