This is so funny. When i was teaching my 5 yr olds to freedive they had the same problem. I couldn't understand it for a while, then I realized the problem (and the solution)... You DO NOT pinch your nose to clear your ears. What I do is use 2 fingers to push the bottom of the nose pocket upward so as to block off the ends of the nostrils.
When I learned to dive there were some oval masks with no nose pocket at all. Some people even glued a piece of neoprene wetsuit on the inside on the bottom of the skirt to make the nose seal easier, but this was before silicon masks.
Just teach him to block the nostrils not pinch the nose.
BTW, I really thought this was common knowledge? If your hands are full on descent, you simply press the mask into your forearm, seal the nostril and blow... no need to use fingers or hand..