After reading his characters biography. I noticed that he was suffering the same depression that Jack was suffering from. He was a drunk and had his medical license suspended for performing surgery under the influence of alcohol. He had spent time traveling away from his family, dissapearing for long periods of time. I think that he had been on the island before. While trying to find his way back to the island he had a relationship with Carole Littleton (Claire’s mom). Then he finds out that he needs to get a group of people together on a plane and get them to the island (not sure how he finds this out). He is able to do this and those people board flight 815. His son Jack boards to bring his body back to the states, which is how he is able to get back to the island (as evidenced by the fact that he has had contact with various survivors of flight 815). I think that we will see that he will have an increased roll in the show. He has far too many connections to all the characters to just be a bit part or pawn in the mix.