But it's not about his past! It's about the present, in which he continues to ignore a little son who has been brought up in difficult circumstances. The PPs saying he was vulnerable, may have blocked things from his mind because of trauma etc, are talking about the 17-year-old boy he was in care. And fair enough. I have some sympathy with the 17-year-old.
But 11 years have passed and he is now a grown man in a steady relationship, and with a second child. It's his attitude now that disgusts people.
He knows he has an 11-year-old son who's never known a father, just had had a string of men staying a while with his mother and moving on. His paternal family are all equally negligent and his mother's family seem dysfunctional. This upsets me, and I'm only reading about it!
He could still help that child, and even try to build a relationship with him. I wouldn't blame the boy if he told his useless father to go to hell, but you could make progress with perseverence, counselling, generosity, patience. It's not optional, in my view.
I think the OP hopes to make him start doing something for the child. But in answer to her question is a bad person always going to be a bad person? I'd say he wasn't necessarily bad at 17, but he certainly is bad now and intends to stay so.
OP should also think how this selfish, neglectful man would behave if they have a child with special needs? If she herself became ill or disabled?