Your boyfriend doesn’t want to lose you but doesn’t want to start over as a parent. You don’t want to lose him but do want a biological child. That’s an impasse that is very, very hard to get past.
I do think you are overstating the problem. He hasn’t said he doesn’t want any bonds with you or that he is rejecting you. He seems to be as upset about the impasse as you are. To characterize it as if he is saying “my way or the highway” is unfair since you are essentially saying the same thing. That’s what impasses are about.
There’s no getting around how painful such an impasse can be. From what you report, he really doesn’t want to add another child to his life. On your side, you can’t accept that your full time stepchildren are the children you have.
I don’t have any easy answer for you. I would only ask you to consider this: No child should come into the world unwanted. From my point of view, you should only go forward with having a child with this man if he fully embraces being the child’s parent. Otherwise, it’s a set up for problems in your relationship with your partner and among all the relationships in the family in the future.
I do think you should consider seeing a couples therapist. I only have the barest outline of the issues to go on. A therapist can hear your whole story and perhaps offer new perspective. It may be that a therapist can help your boyfriend resolve the pain he has felt about his first experiences as a parent. It may be that a therapist can help you turn your maternal instincts to the children you have.
I don’t know how it will turn out. But I do know that any responsible therapist won’t jump to a conclusion about what you should or shouldn’t do. Instead, the therapist will help the two of you decide.
I wish you well.