It's possible that it initially focused correctly, but then between that time and when the shutter was released, the tire moved slightly closer to the camera... If your son was putting a lot weight on the tire, it might have been swinging slightly, which would be why his sweater is in focus instead of his face. One other thing I can think of is if your camera is using multiple autofocus points (instead of just the center one), it might have focused on the wrong part of the subject despite you putting the center dot on his face. Besides the focusing issue, I think it's a nice shot!