Unique situation

Gonna jump right to the point here. I am a 32 y/o male, and was in a 7 year relationship with the mother of my son (biological). She has an older child who is 7 years old (anyone doing math can see where I'm headed), and was of her ex husband. The ex husband saw the child as an infant 5 times, then none from age 1 until he was 7. Biological father made a brief return when the boy was 7 (which totally shook things up, bc the child never knew of him and had only known me as his father). Now the biological father is saying he doesn't have time for it and will not be a part of his life in the future. At the same time this is happening, his mother and i are splitting. I want to adopt my 7 year old stepson. I doubt getting consent from the biological father would be difficult, but the mother may actually be the roadblock. There is absolutely no ground to say my adoption would be anything less than in his best interest, but she may stop it out of spite, and to avoid more child support and custody hearings. I have considered the child in this as well. Is it desirable for HIM to go through that? I want him to be my son LEGALLY, as he is in my heart and mind. This way if his mother continues to jump from one "daddy" to the next, both my biological son and stepson always have a dad they can count on.... The question, is this EVEN possible?