You "play" with him. Like trading, battling each other. After you've done that for a few times you get a choice: "Do you want to add (friendsname)?
You both take your 3DS's and click the orange smily on the top counter. Then you can add, depending on whether or not you are together, each other by Interner or local. Again this depends on whether or not you are together.
Le source(s): Experience.