About two weeks after, he did the same thing again, but this time he used only the back side of each puzzle without looking at front side. There is nothing on the backside except different shape.
It seemed that he memorizes the shape and location of each country and also he understands the concept of opposite direction of mirror image.
Ohhh... my DD9 used to do that when she was little! I'd forgotten about that. Wow. I can't recall how old she was - maybe 3 or 4. She was reading at 3 like your son.
At the time I just assumed she was fitting the shapes together and that it would have been easier without the distracting element of the picture on the puzzle. Any puzzle she came across, regardless of piece shape or size, she delighted in re-assembling from the back side.
She doesn't seem to have an eidetic memory now (unless she hides it - but I doubt it). Who knows with your son though. Maybe he does? I wish I knew more about it. Good luck in finding out
