They're pushing him and you towards the more expensive boards/bindings. Those aren't what he needs. Hell the Legacy is more all mountain freeride than freestyle, so why they recommended that is beyond me. My guess is they're morons.
At that age I highly doubt he's really a 13 boot, maybe a street shoe, but not a snowboard boot. He's just going off how his shoes should feel and not how a boot should actually fit.
Where does he ride, where do you live, and what are your other options from the stores around you or are you willing to shop online? I know we're down to the wire for shipping online, but it might be a more viable option than dealing with the incompetency of your local shop.