Wow...we are in the same boat. It's tough because we're just the "significant others" and they're not our kids...the bio-parents become so defensive of their children's behaviors if we try to point them out. My bf has broken up with me in the past because I wouldn't live in denial along with him when it came to his kid's addictions.
All you can do is take care of yourself, protect yourself, and continue to encourage your own son down the positive path he has chosen.
It's hard...I know. The whole stepfamily dynamic is hard enough on it's own...throw in drug addiction and it's a nightmare.