In my experience, and what I have learned from Romana, the mat work is the best thing for young growing bodies. Since the mat work has an order and is a system in and of itself, I would start him with the basic mat and advance him to the intermediate as he progresses.  Kids love the matwork, it has just the right balance of rolling, stretching and strengthening and has an element of playfulness; in part because of the transitions between each exercise.  If you are not familiar with the classical order, I highly recommend getting the book by Joseph Pilates "Return To Life Through Contrology" and possibly the Dynamic Mat DVD from Peak Pilates which details some of the key concepts and gives you a visual on the exercises.  GOOD LUCK and let us know how it works out.