It's hard to tell without seeing in preteen ball if the walks are a result of a good eye or the control of the pitchers. If he's drawing walks because he can tell a pitch is an inch off the strike zone it's a reason not to change. It doesn't matter he isn't hitting for power since he hasn't grown. Is he hitting line drives? Hitting quality y line drives would be another reason not to change.
A reason to consider hitting left is if he's fast. Given I hit left and was fast I lived the benefits. Given my wife was a track star I assumed the kids would be fast. I taught them to hit left from day one.