Another HYPOTHETICAL situation. Could be 8U, 11U or 13U. Pick one. Or, make a general point that covers all three. The main issue is that we're discussing the first age that a player faces live pitching on a NEW SIZED FIELD for him...meaning "not coach pitch" 46/60, 11U 50/70 or 13U 60/90.
In terms of mechanics, the kid is fine. You have seen the slo-mo video. You have seen the stills. Yes, he's not perfect. (Who is?) But, it terms of hitting "mechanics" there are no major flaws and the kid is pretty spot on in terms of the swing, for a kid his age.
Yet, the results are not there. It's not consistent hard contact. He struggles getting the ball out of the infield.
If you had to guess as to what the problem is, what would you say? Timing? Vision? Lack of physical strength? Something else? Usually, what is the main culprit for batting woes when it's not mechanics? (Again, when the player is first playing on a given sized field.)