There are plenty of "bad" things your son could get into. And cigars are not one of them...
My son began smoking cigars when he was 18 - at least "officially". He and I have had some GREAT conversations over a cigar. He's 24 now, and some of the best moments I have with him are when we both slow down to contemplate life and love over a nice cigar.
My recommendation - make each and every cigar smoking opportunity with your son as good as the last time. Smoke good cigars in a good atmosphere. Teach him to appreciate the subtleties of the various brands and vitolas. You might even be daring and introduce him to a dram of a good Scotch or Cognac - teaching him the value of enjoying the pairing of a good cigar and a fine spirit (rather than smoking a cigarette and chugging down shots of alcohol). Through those interactions he will come to appreciate positive stress reducers and good adult conversation.
Good luck!
BTW - one of my BEST father/son times was when I took him to Las Vegas for his 21st birthday. The brat decided to drink a "yard" (e.g. 10 shots of alcohol) in less than an hour. I spent the rest of the evening holding him up by the seat of his pants so he did not fall flat on his face... To this day - one of the best stories I tell in his presence.
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