Hi There...
Further to this (having spent 2 seasons as a physio in the ski slopes of Canada and also breaking my own wrist in the process)... the snowboarder type wrist guards are a good start. they just protect the wrist from hyper extension when falling. it's not worth the $$ to get anything bigger or fancier because it really won't do much more. It's also important to explain to your patient that if he's falling to clench his fists and tuck elbows into his side at 90 degrees-ish which really reduces the chance of falling onto an outstretched hand and hyperextended elbow, and the bracing for the impact is also heightened with muscle activity to clench the fist. i would also strongly suggest he get a helmet (sounds accident prone) but that would also help with teaching him side impact fall/roll (downhill skis will release to enable this) and protect his head.

hope that makes sense!

msk101