It is illegal to buy for or give cigarettes to an U18.
“I know men that would buy their 17 yo son a pint of beer, doesn't mean they are turning them in to ragging alcoholics. A ciggy isn't much different. “
Actually ‘a ciggy’ is very much different. The legal position is different due a start. The vast majority of social drama makers do not become problem drinkers. Everyone I know who smokes daily (as the OP says her Ds had been doing with her DH) is addicted. I. E they find it very very hard to give up.
Many people believe, with plenty of anecdotal evidence, that introducing older teens to moderate, controlled drinking in a family context stops them going mad as binge drinkers. No one thinks that a bit of Father and Son smoking together helps young people not become smokers.
And smoking and moderate social drinking are way different in terms of health risk.
OP, In your shoes I would tell DH that either he instigates a joint and shared ‘let’s stop smoking together’ initiative with your DS or he gets the hell out.
And never let anyone smoke in your home again.
My DH smokes one or two cigarettes every evening. He has never ever smoked in front of the kids.