I think I understand about feeling guilty. You like her and don't want to hurt her. At the same time you must know that a mother involved with drugs is unable to take good care of her child. If you think that she's doing an adequate job then you should rethink taking him from her. But I've had a lot of experience with drug addicted mothers and I know from first hand experience that even tho they love their children the drug is ruling their life and they cannot care for anyone, not even themselves. This isn't always obvious. It may look like she's doing OK but she's not. The longer you wait to take the son the more damage he will have suffered.
Has your boyfriend talked with his ex about her relinquishing custody? If she is willing I would go thru a lawyer to have a legal document drawn up so that she will have to stick with it. And perhaps she would be more willing if she believes that she still has a chance to be more of a mommy when she gets rid of the drugs. Not necessarily that she would get him back but that if her situation is such that she can be trusted to take good care of him she could have him for more time.
This would be better and less expensive than a court battle during which your husband and his attorney will have to prove that she is unfit. Since she's been arrested twice it seems obvious but she may have a good attorney who will be able to prevail. My cousin and her husband went to court several times to get custody of his two kids. She was arrested several times but was able to retain custody by accusing their father of being abusive. I'm sure that their is more to that story but I know that the courts do not always rule the way that we expect.
As to whether or not you should become his mommy because you're not married I'm thinking that you already fulfill the role of step-mother. In that regard nothing is changing. This is your husband's battle but as his partner you are in reality involved even tho you aren't in legal terms. And you already are an important part of his son's life.
I have read your posts and I'm confident that you can do this. As the other mother said you love him, play with him, take care of his physical and emotional needs just like you do with your son. You will feel a different kind of love but that's OK. Even birth parents do not feel the same way about all their children. I have adopted my daugher. She came to live with me at 6. I don't think I love her in the same way I would've if I'd given birth but I still love her immensely. I have stayed with her through 20 years and those years have been tough.
No matter what you do he will feel like you favor your birth son. My granddaughter is sure that her mother loves her younger brother more than she loves her. That is just the way of siblings.
Life will be more complicated for all of you and it's good that you're hesitant. That means you are aware that there will be difficulties and that puts you in a much better position than someone who just says she'll do it without considering some of the things you are thinking about.
Try to relax and treat William as the little boy that he is. Love has no limits. There will be enough to go around.