Hiya
IMHO (and it is humble, because I'm not in your situation and you must be worried sick) I wouldn't try to get him to stop at all. I would tell him I knew, casually, and I'd let him do it in front of me if he wanted to - I'd be his friend about it.
It is AFAIK very hard not to smoke once you are attached to it (for want of a better word) and it must be important to him if he is continuing to do it.
Therefore I would not be trying to offer support to stop, or whatever - I'd also not be telling people about it because if he wanted them to know, he would tell them himself.
I would be there for him, smoking or not, and accept it as part of him, as it is. It's his decision and he is still the same person.
The second you start looking like you want him to stop is the second he starts feeling worse about it. And he shouldn't. And it might make it harder for him to stop if he's under pressure from you or others.
Accept it
Don't mind it (at least to his face)
Make a joke of it if you have to chastise him
Don't let it change your relationship or become the focus of conversations
Don't make it an ultimatum or a threat to tell if he doesn't stop
That would be my approach. It's a shame if he feels he can't do this thing which he wants to do, without being told off or whatever.
I bet there is no one he feels he can tell. It must be a lonely place.