I haven't been able to find one that fits my small frame yet. I wear the smallest Snowpulse bag. BCA bags fit a bit shorter in the torso, but not as comfy. If he carries it empty it might be OK for him.
You may want to check out WARY's avalanche vest. They are available in different sizes, offer chest and shoulder protection too. My wife wears the Snowpulse avy vest, and finds it far more comfortable then a backpack. I like the fact that it has body armor built in, so serves two functions. (I wear a tekvest with my snowpulse highmark 22 pack)