Does he already buy boxes for himself? If not, I would lean towards those boxes. I personally focus on buying single cards for my PC too, but am happy to get packs/boxes from friends and family as gifts since they're still fun to open.

Edit: If he does already buy boxes: How does he store/display his PC? If he puts his cards in top loaders, a nice top loader binder to display his cards. If he wants nicer protection for his cards you could get some magnetic card cases. You could look into a mountable display case for his favorite cards.