Okay mou.
I worry though, that perhaps you are giving your son a responsibility that is too great. I think, to him, it will appear that you are asking him to choose what happens next, by asking him what he wants or by involving him in your decision-making.
I think it would be good to bear in mind that children often don't say what they feel, because it terrifies them, or they feel that it would jeopardize what security they do have.
DS may want his old dad back (of course), and at the same time be desperate, without even being able to admit it too himself, for you to show unequivocally that you will defend him against anyone who threatens him (emotionally, I mean). And that for you, he comes first.
He will never, in any case, get his old dad back. Even if your husband's behaviour improves, the old dad is the one that never said 'have him adopted.' It's not possible, ever, to get back to that dad. It's only possible to 'move on' from what his father said, and moving on is not something, I don't think, that a child can be asked to do. When he is more mature he can decide for himself if he wants to move on.
Children are also often scared of change. He will also feel that the weight of the whole family's happiness (brothers and sisters, and your own) rest on his shoulders.
It's possible, also, that you are telling him that you will leave if he wants it, but giving out all sorts of other signals that you don't really want to do that.
I feel a huge weight of responsibility, coming down with such a strong opinion when a family's future is at stake. This is what I believe, and firmly, but only you know what's best for him and you all, and I wish you very well.