You don't have to make any decisions TODAY about how to handle this situation. Sit on it for a while and pray for guidance. Whether this child goes back to his bio. family is out of your control completely (unless, of course, there are substantial reasons why he should't be there). The most important thing you can do is let him know that you love him, that you'll always be there for him, and that your home and heart will always be open to him if he has to return to his biological family. Its not an IDEAL situation...not by far...but in this situation, it might be the best thing you can offer. Don't underestimate the influence you have had on this child's life. It sounds like you've given him love and a stable home environment. You may not be able to prevent him from having to leave, but you can remain a stable, loving force in his life.
Open your mind to the possibility of talking to the extended family about what's best for the child, preferably in the company of a child therapist who can sort out the goods and the bads of all of the different behavior that's gone on. Tell the therapist that you'd like to walk away with a road map to minimize the damage that's been done already, and make the next few years of his life better.
Lies, I'm so sorry you are in such a difficult situation. Not only are you dealing with the pain and anger of a disease gone out of control once again, you are worried about a child you love. In situations where emotions are so raw, I find the need to live in today to be of utmost importance.
So perhaps, just for today you can love your step son unconditionally and be there for him to get through the difficult time of coming to terms with his dad's relapse. Are there any Alateen meetings in your area? This might be helpful to him...to share with others his age that are going through similar tough times.
I think there are so many wise words in this thread. I particularly focused in on what OOAL and Give Love said above. There is no need for a specific decision today as long as this little boy is safe and loved and feels secure. I think that there are so many things to weigh, but that also the child is old enough to have some thoughts about what is going on. He may not know what is best for him...heavens, sometimes we as adults don't know what is best for us when we are in the middle of a crisis...but with loving support, honesty and time, a solution will work itself out.
I'm sending prayers that your HP gives you strength and clarity and that you will know when the right time is for the next decision to be made. Hugs