I used to BEG my mother to quit smoking... and now I'm a smoker (trying to quit! Doing really good!)
They say that MOST people start smoking if their parents were smokers (even if they quit, like in my case), or their best friends, boss, spouse, or other close family members smoke. OR they start smoking due to peer pressure (which would have been over by 19, you'd think) or when they're under extreme stress.
Seems like he's basically a pretty responsible kid, works, bought his own car, not too many people have 19 year old's who can say that! So admire the positives; there are obviously many.
Don't think of this as you or your husband somehow failing as parents... my parents STILL preach the evils of smoking to me, and I've been smoking for over a decade (ouch, never thought of that before)... and remember, it's every cigarette he DOESN'T smoke that counts. NOT trying to justify it by any means, but being a social smoker is better than being a 2 pack a day smoker... not by much, but there's still hope, that's all. He's not addicted to the additives if he's only smoking 1 cigarette here and there.
I'd guess there's something psychological going on here, so maybe instead of addressing the smoking, see what else has been going on and ask him if he needs help with it.
Sorry to hear this though, I know it's got to be a bummer for you and your husband. Feel free to nag him about it too, it WILL stick in his head! Good luck!