Having been an overweight child, I can really feel for your son. I would probably side with the consensus to not just buy the JC food for your child without consultations.
However, I am wary about just waiting until he is 13 to start getting healthier. I was doing some digging and came across a couple of things.
One is a website,
http://www.drsearslean.com/, and the thing I liked about Dr Sears site was that it is more about getting healthy and fit, not just losing weight.
Another suggestion I found was a video, apparently Leslie Sansone does a walking video for children as well.
Good luck to you and your son.