It's been internally fixated so the chance of him re-fracturing in the same area is very unlikely, though it'd be painful for him to land on it thats for sure. I suggest doing some fall training with him however - slowly building up height so that he becomes comfortable with it. And a very good idea to help strengthen that bone.
Rigid splints will mean when he falls he'll take more force in the areas that aren't in the splint... so more likely to break a bone in the hand for example. a more moveable splint would be ok to limit extreme's of ROM but won't prevent him breaking it too much. The proprioreceptive benefit of wearing a simple wrist brace from the chemist may be sufficient to help him prevent a break.
Do some fall training with him and maybe someone else who ski's (as I don't) will have a better idea on bracing for you.