D&D is huge now and getting huger. Whether the adcoms know this or not is an open question. Currently D&D (not the gaming company but the gamers) is producing full-on podcasts of actors playing the game (hilarious I might add); there are board-games (general) and various fantasy board games as college clubs forming right and left. You may want to check out the college’s clubs page to see if any gaming is cropping up there formally. Even if it’s not, he could mention that gaming is big and he could help start it. Players of D&D know that it’s great for writing and art skills (teaching plotting, character development, etc).
Board Gaming is getting so big that now there are several cafes and bard in Manhattan that are devoted to this. They are packed.
It’s a “thing” and it seems to be growing still – going mainstream. If he were my child I would help him create a resume in addition to the Common App. Not all schools allow you to submit a resume but some do. I would make sure that gaming was on his app and would ask him to write 1-3 sentences explaining how D&D affects his life and his plans for it at school. This allows him also the ability to “speak” to the adcoms in a way that he may not be able to elsewhere on his app. Just one more chance for them to get to know his wonderful personality.-- and maybe get hip to the burgeoning gaming culture besides. If they see that this will add to their community, that will probably interest them in a positive way.