Jen,
My little boy went through that also. He is 4. My husband finally quit smoking a few months ago(So proud of him) but the influence was there. I have reiterated to my son how bad smoking is for you. I really don't think he understands really what it is. But, I've found the bigger deal I make of it at his age now, the more he dwells on it...I talk about how bad it is briefly and then divert his attention.
GOD BLESS!!
Jamie