last time I was there.

Folders for your character sheets. Containers for your dice. Cool mats or trays for rolling your dice. Like others suggested, figurines of the characters you play (very useful during attacks--helps you keep track of whose attack it is and who's within striking distance of enemies).

If he has a favorite class, you can ask at the game store about special items for that class. For example, there are decks of spell cards--convenient if you play wizard class.

If the party he's adventuring with gets into their roles, you could consider something that goes with the character he's playing — like a wizard hat or a cloak.

There are tons of T-shirts that have D&D slogans.

If he were DM-ing, then I'd suggest the dry erase mats that let you sketch out your dungeons so it's easier for players to visualize their attacks and the placement of their enemies.

Graph paper — little squares (like 4 to the inch) and extra long pages (like 11x15 or bigger; you can find it at Hobby Lobby) is also convenient for keeping track of what you've explored and where you are.