To keep talking about it.
That kept bringing it up.
Let your fiance bring it up if he wants to when he quits.
Otherwise, let it go.
And, maybe, give a weekly treat, "just because."
On another note, though, if the depression came to the forefront when he quit,
he should get some help for that.
It's quite common that smokers have an underlying mental health issue. And also, when we quit smoking, we are loosing a very dear friend, who's been with us for years, through thick and thin. So, depression can *start* as a result of that quit.
If he decides to quit again, he should ask his doctor about the depression. There are medications that can help him over the hump. Especially if it's just the situational depression resulting from quitting.
I hope he can find his way.
Perhaps you can send him here for support when he's ready.
There's lots of wisdom to be gained by sharing our collective ESH.
Shalom!