Graci-
Great advice before me.
I have to agree with lightquest. In a sense, it is not your responsibility to keep the conversation light- addiction is very serious and can ruin both your lives as well as end his.
In my experience, someone must hit a bottom and live in utter hell until they are ready to get the help that they need. His enduring the pain of this moment may be the best thing for him in terms of his willingness and ambition to recover and live a full and healthy life.
I know it is hard and you want to be there for him, but remember yourself.
I do not know if he has been open and honest with you about his addiction the entire time, but I know for me my abf hid so much from me, lied to me time and time again and deceived me to hide and use in peace. We all suffer from a loved one's addiction.
I always found myself telling abf what he needed to do, how much he deserved a bright future and how much I wanted us to be together in that future. I honestly believed our love was enough. Sadly, addiction is so much more powerful than that- a gripping, lethal force. I never thought I could loathe a disease- a non-entity so much.
I think it's pretty hard to keep the conversations light. I would encourage you to tell him you are there for him and believe in him. That is what I try to do and many times when I catch myself going off on some lengthy inspirational speech I have to stop myself and remember that nothing I say or do will matter unless he TRULY feels it and sees it for himself.
Feel free to PM me anytime- I understand. SR is here for you. This is an incredible forum that has taught me SO much.