@45thighs123 I'm actually in the position of your son.
My dad was in a v similar situation to your fiancee, there was contact agreed in court but the mother and her family frustrated it. It was a long, long tie ago and enforcing rights of unmarried fathers seems to have not been done in the way it is now. I'm a lot younger than my half-sibling and because contact had totally broken down before I was born I wasn't told of their existence till I was an adult.
This will hugely affect your child. Almost no matter what you do. They'll wonder about the siblings they don't know, they'll wonder if their dad would leave them if your relationship broke down. They'll do family trees in school, people will ask about their sisters, they'll ask you about why they don't see them. Maybe not as children, but as adults. SO much of how my father parented me was, in retrospect, linked to missing out on his other child. I still judge him, a bit, despite him being an incredibly engaged and involved father to me, and thats in the context of knowing that he genuinely pursued all the legal options available to him at the time. Knowing what it is like nowadays, I think there is every possibility that your son will hold it against him in twenty or thirty years, and it will affect him significantly.
I would struggle with staying in a relationship with someone capable of walking away like this. Also - is the fact you have a son and he walked away from daughters a factor in his thinking? In your case I think you need to make it clear he needs to keep doing something. I am sure you can self-represent in court. He needs to be able to show to those girls, and your son, that he did everything possible. They are far too little to give up on at this age. Even my dad fought until his child was old enough to tell him they didn't want contact - it was complete parental alienation but he had no choice.
You are talking about him, and the girls. You haven't said much about your son. My parents didn't really think about how their choices would affect me - they basically took a half sibling away from me. For your own child, you need to get your husband to fight harder. Or you will have questions to answer from your own child in the future.