A trick that used to help me when fogging started to show up was pulling my lower lip (and teeth) in a pit and bringing my upper lip out and down a bit, so that exhaling through my mouth would go down, rather than forward toward the shield, then inhaling through my nose and exhaling through my mouth. Might not help in your son's case, but worth a try.
I didn't know Rain-x made anti-fog wipes, but back when I still needed to wear a helmet, they sold anti-fog fluid in the same type of small squirt bottle that regular Rain-x came in, and it worked very well. Might look and see if they still market that.