Everyone has their own opinion when it comes to controlled substances, but my general rule of thumb is it's their business until it interferes with something. Not showing up to practice or gigs, being out of it during the writing process, screwing up a performance or something like that. You can voice your concerns to him and just be honest, without demanding he make a lifestyle change or putting a personal judgement on him. If getting confronted with that is a bit of a wake up call to make some kind of change great, but if it's not you're not going to be doing yourself any favors by trying to drag him kicking and screaming to a lifestyle change. You don't even have to bring drug use into it, just tell him what he's doing that's negatively effecting the band and communicate with the guy about it. At some point you've got to be willing to let him go though, if he's negatively impacting the band or even just you guys personally.
Just my opinion, interpersonal relationships in a band and dealing with people who are deep into controlled substances are sticky situations all on their own.
Just my opinion, interpersonal relationships in a band and dealing with people who are deep into controlled substances are sticky situations all on their own.

People that are SOOO against drugs fail to see that fighting the war on drugs only adds to the problem. 