Since my dad's diagnosis my sister, sister-in-law, and prior to my dad's diagnosis my brother have quit. It has been a difficult road. This is an addiction that I have never had, thank goodness, and therefore one I don't understand. However, it seems to me like for the quitter, it was their life, their best friend, and they are giving that up. I have found with the others that they needed new best friends. The focus of their life (though subconscious) is gone, and they need it replaced. I would do something special for him every day. Ask him how he feels, leave him notes, buy him inexpensive gifts, and make it a focus for a while...especially as it is so difficult.
Of course, I don't know enough about this, but I can say these things have happened around here. My sister-in-law says she needed us by her side to replace the cigarette. She is pretty open about it, and that makes it easier. Plus...we all know men... we always have to read their minds anyway right!! haha!
Good luck to you!