No one can really tell you what to do. I wish it were that easy, but it's not.
You want him in your life just not as his girlfriend. So......you want to be his friend? I'm not sure I exactly understand, but again that would be a choice only you can make. He says he's willing to stop smoking if he can have you back in his life, but he's not saying he wants to stop smoking because he wants to. Until he's ready to stop for himself, he won't stop........he might get better at hiding it from you, but I don't think he'll stop. I once thought love was the cure, but was quickly disappointed when my exabf just couldn't stay off the crack.
If you truly want this man gone, I suggest you change your number and cut off contact. If you are tempted to talk to him every time he decides to get a hold of you ..........cut off the temptation by cutting off any type of communication.
Hang in there and be patient with yourself. I took my exabf back more times than you can count on all your digits until I finally had enough.