I have both a full face and a regular helmet. So lets look upon it from two perspectives:
1. What is your kid stoked on? Mountain vs. park?
If he will keep on charging lines, cliffs and doing that more and more exposed, then a full face helmet has its purpose. Against rocks and hitting other solid/sharp object. But when it comes to all mountain and park, i would say the oposite. A full face helment, despite of what has been said above, it reduces the view quite significantly, it also ads weight to your head, reducing both coordination and the feel of/freedom in rotations. You then have an object that will create drag. A full face in the park is a bitch, to say the least.
2. Possible dangers from wearing a full face helmet/positive sides to a regular helmet:
A full face will protect against rocks and such, yes, Its made for HIGH impact against hardt objects. But it will be in the way in the park and add risks for neck injuries. No need to get a hang up on the "snapping" of neck, a hefty tear of tissue/disk can be be enough to put him out for a few weeks+.
A regular helment and an exposed face also has some decent bright sides! Our bone structure is made to absorb impacts, and free sight and noting in front of your face can't be underestimated. It is a huge difference unless you are in terrain where you worry about rocks and shark-fins in the snow.
PS: Wearing that neck-collar mentioned up there is just plain bullshit if the fit isn't correct. Wearing tight body-suit wests (and collars) like that reduces the flexibility in the chest and body, making your body more static, increasing the rist for fractures in both neck and back. That being said, i wear my back brace, a little loosely, all the time, also when not skiing with a helmet. But never anything that will obstruct the flexibility of my chest.
