I'm confused by the question. Gloves come in many lengths, as noted above. But when you're talking about the inside (hand) size, usually there's the youth size and the adult size. The inner size doesn't get smaller when you get a shorter glove, nor larger with a longer glove.
I've never known anyone to have hands are so massive that a standard adult glove feels uncomfortably tight, but if your son is the first, I'm thinking there must be precedents at the pro level. So maybe check out web sites for Rawlings, Wilson, Mizuno, etc., or use their "contact" page, to ask about gloves for mountain men.
Beyond that, I've always thought the Rawlings models were more narrow, Mizuno in the middle, and Wilsons were larger in the palm. My own hands are pretty average, and I always felt uncomfortable with a Wilson (e.g., the A2000) because it felt like my pinky was getting spread too far away from my thumb. So if maybe your son has been using Rawlings, he may do better with a Wilson.