Lies,
As one who was raised in a household with addicts, my opinion has to be taken with a grain of salt.
This child needs to have whatever home is going to offer him the maximum love and stability and freedom from addiction's madness for at least the next seven years of his life. Like Dolly, I don't see it as a huge crisis that the extended family told him THEY were his parents...my intuition says they were just trying to make it all less confusing to him as a lil guy. I also (forgive me) don't see it as a huge crisis that they tried to encourage him to NOT be with your husband. People outside of addiction can't see why people like you and I would stay with an addict, and they see things differently than we do.
I wish someone had offered me an alternative to my situation when I was 11.
If you love him, if you are truly committed to his happiness even if it means you will eliminate all unnecessary contact with your XAH (if you follow through and do this...do you know for sure that you will? Ask any of the wives here how easy that is to do), and if you will protect his peace of mind and stability no matter what, then consider keeping him under your wing.
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.
Consider gathering the courage and integrity to admit to them (as you've done to us) that you guessed wrong about your husband's addiction, that you're sorry for your inadvertent part in all the chaos surrounding this poor kid, and try to find a solution TOGETHER that is best for him. Sometimes, when we put our heads together with people who used to be the enemy, we can come up with some good ideas.
Good luck with this.
Hugs,
GL