i have a burton and an airwalk, and like the airwalk much better, but that has more to do with the specific model and size than the brand. but the nicest board i ever rode was an m3. too expensive, though. boots also depend on his foot shape.
i'm 5'5", 115lb and ride a 153 on the mountain and a 149 in the park. ah! that means i'm probably the size of a 14 year old boy.
anyways, your son could probably be pretty comfortable with a similar sized board.
but you'd have to say if your son was a beginner or advanced, if he liked the park, trees, carving, backcountry, etc to get a better recommendation.
you can get a great deal on used boards if you hang out at a ski resort the day before the employees are paid (or the day before the drug dealer comes). that's how i got both of my boards. people are desperate for money and will unload gear at great prices.