He shoots left of the target most commonly. I can't move his site anymore than it is. He is also not consistantly left enough for me to believe it's his site.
Just a thought....instead of trying to move the sight so far over, you may want to try moving the rest a little bit over instead and line everything up. Only thing I can suggest is shoot at 10 yards or less until he gets that down and then move back to 12-15 and then get those shots and alignment down and keep moving back. Good luck!