Julian,
I coach a team at Ski Roundtop-- and we have had three of our kids between the ages of 10 and 13 make the switch to hard boot setups. All of these kids were very proficient at turning their soft boot setups.
The problem, or issue came when they moved onto their alpine setups. What I have found is a tendency to skid due to their legs being straight. Another issue that would concren me, is how much time has the kid been freeriding with that setups outside of the gates. You can learn very little in the race course about riding. Sure, there's timing, edge pressure and line to be taught in the course, but not a clean race turn or a carved turn, that should be left for the open trail. Make sure you kid is comfortable carving first before throwing him into the course.
I would venture on to say your son is probably "swinging" his upper body to make the alpine setup turn. It's a natural reaction when placed on a stiffer setup. I would have him do drills that prevent "swinging." For instance, he could do a drill Phil Fell talked about by riding with his hands on his hips. I have used this to show the kids jsut how much they are swinging their turns instead of pressuring the tongue of the boot and driving with the lower body.
Additionally, I would have them ride with the left hand on the left side of the board, and the right hand on the right side of the board. this will help square up the shoulders and make sure alignment is correct. A drill that works is have the kids ride with ski poles for a run or two. I try not to keep them on poles all night, as they become co-dependent on them, but again it helps with alignent and even timing if you have him reach forward with his arm and pole plant like a skier would...
Once alignment is proper, you can move to angulating with the knees, and hips while maintaing an athletic stance with the arms out, and straighter back to prevent bending at the waist.
Finally, I think your problem will be alleviated as "junior" grows a little more and is able to decamber his stick, thus allowing a carved turn more. A drill I have our kids do is a compreession and extension drill... ride across the hill on the uphill edge, then at the moment of turning, pressure the nose of the board, and compress to decamber the board. As the turn is completed, have thme stand tall to help "unweight" the board allowing the next traverse to begin. Repeat until you've logged many miles- kinda like this reply. All the while, you can be checking body positioning - arms, shoulders, angulation, etc. I have some of our riders hit a slight mogul of snow pile to do and edge change, as they become slightly air-borne. this seems to allow the feeling of a decambered board and allow them to feel how a weightless (as in decamber) board will feel as it cross under the bod y for the next turn. it usually starts from toeside to landing heelside and proceeding witht he turn. this air-borne turn helps when landing for the kids to generate a little more weight and force into their heelside turn and bend the board.
I hope this helps. feel free to email me if you need any more ideas. neilsunday@aol.com Good Luck, and don't worry. he's 12- by the time he's matured, he'll be ready for the world cup!