Oh dear, first please know that at one time I too was "the great codie detective". Then I realized that when I found information like that, it bothered me and triggered the worst in me. It took time and a lot of fretting but I stopped doing that and began trusting my instincts instead.
So, said with love in my heart...
If he is behaving at home and maintaining good grades, if he is improving with his recovery...then I'd leave it alone. In the end he is the one who will have to make hard choices, like the hot chick who sells drugs.
Give him the dignity of choosing...even of making his own mistakes, that's how they learn.
Added to love, tears, threats, anger, pleading and more...add that detecting has never changed the outcome or made anyone get clean that didn't want to.
Trust your instinct. If he shows signs of relapse then make your boundaries clear once more and stick to them.
Hugs from a mama who's been there, done that and bought every t-shirt in sight.