Christian could quite easily be alive, because of Sawyer. Who might have never met him and things could be different on Ana Lucia's end also.
There was a mention of a phone call in Lighthouse from Jack to David. Which Jack still ended up hearing. This is similar to the phonecall Christian was supposed to make in "Outlaws".
Unless there is huge twists with David's character, he is simply Christians substitute, for Jack's father issues.