It is a fine line, you dont want him to think you dont trust him....
I beg to differ, the less he thinks you trust him the better. An addict's mind thrives on how much of your emotional constitution he can manipulate. He counts on you're ability to cave in and help. His conscious mind may not be even aware that he does it like this, but he does. I said it before, and it may sound insensitive, but keep your distance. A practicing addict (one that isn't even attempting to get recovery) is dangerous to your ability to see things how they really are. Try some al-anon, get some "good" information on the disease and how it operates. Then you'll see your son as he really is.
Good luck ! God Bless, Spud.