Why does he have to try so hard to hide it from you?
You are his wife, not his mommy... If he feels like he needs to smoke, then let him smoke.
Go ahead and tell him how you feel about it, and be supportive of him if he makes an honest effort to quit. You could also set ground rules, such as no smoking inside, in the car, brush his teeth before kissing you, add to life insurance policy, etc.
But I feel that making a huge issue out of it will do your marriage more harm than it's worth. Already, it has fosterd a setting where he is keeping secrets and lying to you (if he truly is smoking again...)
I got my husband to quit smoking about 4 years ago... But he still snags one or two every so often. I give him the stink eye, make a sarcastic joke, then move on.