If both of you have the game and only one has the DLC, the DLC is not shared.
He keeps the DLC and you have to buy it for yourself.
From Steam family sharing FAQ: http://store.steampowered.com/promotion/familysharing?l=english
Who owns and can access the DLC and in-game content associated with a shared title?
A guest will have access to the lender's DLC, but only if the guest doesn't also own the base game. Guests may not purchase DLC for a base game they don't own. Any player may purchase, trade, earn, or otherwise acquire in-game content while playing a game, but in-game items cannot be shared between accounts. These items remain the property of the account that purchased or acquired them, whether borrowing or lending the base game.