Like others have said, it can be used to cut back his smoking. He doesn't have to quit. It's technically more switching than quitting... Switching to a MUCH less harmful alternative. Being a dual user is fine, it's all about the ones you don't smoke. A full switch is possible, but I wouldn't push it too far. It's your dad, so take what you can get

I'm just saying the conversation doesn't have to be about "quitting" more of a "like smoking but without all the tar/rat poison/etc" so I guess, smoking with all the benefits of quitting - increased lung capacity, taste and smell return, less risk of health problems, no longer smell of cigarettes... If he does end up quitting.