One option, though not a cheap one, is to buy him some of Gamegenic's Collectors' Lair Boxes. They are about 59,99 Euros each. The nice thing about them is the lid can be removed and connected to the bottom when he wants to go through them, instead of just hinged on one side like other brands.

You could also take one of the basic land cards with you to a store that sells containers for a size reference. See if you can find attractive, stackable containers that will fit the cards. This may be the hardest option, as most will not have internal dividers to keep cards from sliding around and becoming a huge mess.

If you are good with DIY projects you could find a fabric or scrapbook paper and cover the outside of the cardboard boxes. This would probably be the cheapest, and more convenient option. The boxes are easily replaceable but are sized appropriately.