Why the **** would you tell him that you were going to leave him if he lied about smoking again if you had no intentions of leaving him? That is the silliest thing you could do. You cannot control him but you can control yourself so I suggest that you learn to control your need to control and just let him be. He will quit (just like you did) when he is ready. If you can't wait that long then learn to shut up or leave.
Harsh? Yes, but who do you think you are trying to control someone like that?
As an ex smoker I can clearly tell you that the more you bitch about it, the more he will find ways to hide his habit. Just let him smoke OUTSIDE away from you and the baby and give him all the information you can find on how to quit, what aids he can use both medical and non-medical support and anything you can google about the bad affects it can have on his health.
If he's smart then he will use some help to get off of them so that his baby doesn't inhale third hand smoke off his clothes.
In the long run and the bottom line. You bitching at him will do NOTHING to MAKE him quit. Stop being so controlling and be supportive in helping him stop.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion