You can buy them a nice playmat, playmat bag, or dice bag from inked gaming: http://www.inkedgaming.com/collections/playmats

They have good quality product with a lot of art options. Pick some art you think they will like!

Or perhaps a deck box? This one is very reliable, and has good reviews from mtg players https://www.amazon.com/Ultra-Pro-Satin-Tower-Boxes/dp/B00AF8237K/ref=sr_1_18?s=toys-and-games&ie=UTF8&qid=1480520329&sr=1-18&keywords=deck+box

You can get them a bundle or fat pack from any of the recent sets. http://shop.tcgplayer.com/magic/kaladesh/kaladesh-bundle

If you want to be a little riskier, try fancy commander staples, maybe a lightly played or near mint full art cryptic command http://shop.tcgplayer.com/magic/magic-player-rewards/cryptic-command Or a lightly layed or near mint fullart foil wrath of god http://shop.tcgplayer.com/magic/magic-player-rewards/wrath-of-god

These fancy cards are something almost every commander player wants but won't buy for themselves!

There are preconstructed decks for commander, but he may already own a few of those already.

The bundles and the full art foil cards I think would be most appreciated, but us commander players are always in need of supplies like play mats and deck boxes.