When ever my little brother's birthday rolls around, I always buy him something Dnd related. Same with christmas.
Here is my strategical buying list to get a kid hooked to the fantasy genre's crack.
For christmas I bought him the DnD Basic Set. For his 13th birthday I gave him a player's handbook. For the next christmas I gave him a monster manual and a DMG. For his 14th birthday I gave him a DM screen, a module and atlas for kingdoms of Calamar (both free at the comic and game shop I used to work at. I actually scrounged the atlas out of the dumpster.) I also got him Complete Adventurer and three boxes of minis.
He is now geekier than me, though I know the rules better. I game with him in my uncle's campaign and its really fun that there's three of us; we all look alike have the same weird sense of humor. I bet its more than a little creepy for the other players.