I don't think you need to instill fear into your son. Your son should not think that one bad decision means the end for him. I'm all of us, as a whole have made some bad decisions regarding our CF sometime down the road, and we've learnt from our mistakes, and we're all the better. Experimentation is normal in adolecents. We've all gone through a rebelious phase, whether its not taking our meds, not doing our physio, or simply trying a cigarette, or a joint. Don't let him think that one bad decision is the end for him - but at the same time, let him know that whatever bad decision he makes - he should learn from it, and not do it again. Instilling the fear of God never helped anyone, but I think rather makes them more rebelious, because they are told that if they do something wrong, that that's the end. Curiosity and all.
And like you littledebbie - I have formed a very close relationship with my mother. She is my best friend in some instances, and still to this day - even though I haven't lived under her roof for 5 years, and am 24, she still to this day nags me, and is to some point over-protective. Part of that is what keeps me up on my treatments when I'm feeling down on myself. Just knowing that she is behind my back, pushing me along the way, fighting with me gives me hope for a longer life. Because its not just me fighting. Sometimes you just need that kick in the pants, whether you like it or not. I'm very thankful for my mothers nagging.