As someone who has had addictions, I definitely would say that it really depends if you're up for it. He has to want that change and if he can't see his 'occasional' usage of a drug he's had issues with in the past being detrimental to if he's able to succumb his addictions and lead a better life, he isn't exhibiting that he is thinking about his situation. I say you bring it up to him that he shouldn't be taking substances like such at all because his version of 'self control' will escalate over time (or rather he'll rationalize more frequent use before ending back up at square 1 with a full blown addiction), and if he won't let it go, it's in your best interest to take a step back from the situation as it's just destructive to himself and hurts you along the way.

also: if someone said something similar or something I'm beyond sorry, I just didn't get the chance to read through all the replies