"My baby".
It's his baby, too.
The both of you made that baby. Together. He is the father. And that baby has a right to a relationship with BOTH of its parents.
You'll notice I didn't talk about his rights. Or your rights. I talked about the child's rights. Because that's what matters here. There is a wealth of scientific evidence that, unless there is abuse or neglect, then it is strongly in every child's interests to have a meaningful relationship with both their parents. Kids who have both parents involved (even when those parents aren't together), do better on almost every objective measure of their life chances. They have fewer mental health problems, fewer drink and drug problems, fewer teenage pregnancies, higher self confidence....the list goes on and on.
Your ex made a really poor start here (I'm going to set his family to one side for a minute, because you need to view him and them as separate - I will come to them in a moment). He started off keen to be involved. And then it sounds like the panic set in. His family appear to have worried him - that he's ruining his life having a baby so young, or that he's being trapped into a relationship, that having a baby together when youre not in a stable relationship isn't for the best, or whatever else. And so he came back around to the idea of an abortion as the right thing to do. That's very upsetting for you - you'd already made the decision to commit to the baby, so it will unquestionably have been difficult to hear him raise questions about an abortion. He wouldn't be the first father (or mother, come to that) to have those fears, or to take longer to get to the same place you have, and commit to the child. But he is there now. He clearly wants to be very involved in the baby's life. That is a good thing. And it is in the baby's interests for him to do that. It's in yours, too - because sharing the burden of parenting is a hell of a lot easier than doing it all by yourself. And he does have the right to involvement in his child's life - mums who try to shut out the dad because of their OWN feelings are not acting in the best interests of their child. Trying to exclude the other parent from the child's life is wrong. Alienation (which is the process by which some parents try to turn the child against the other parent) is widely reconised as a form of child abuse, because it denies the child that positive relationship with the other parent. Attempting to exclude the other parent from the child's life because of how you feel about them is no better.
So, I would urge you to put your feelings about his early reaction to the pregnancy to one side. He didn't get it right. You were right to feel upset. But he's caught up with you now. He wants to be involved. And so, if you use your earlier upset as an excuse to try and prevent that, he isn't the bad guy any more. You are.
Now is the chance to get your interactions with him off on the right foot as co-parents. I'm a big fan of 50/50 when the child is old enough, but it isn't a realistic proposition for a newborn. So now is your chance to talk to him about what would work. Sit down with him. Make clear that you are committed to having him in the child's life, and parenting together. Talk about how that might realistically look in the early days, and how it might change to expand the child's time with its dad as he or she grows. I'm sure you can get plenty of ideas on here for how you could make that look, if you want them.
Ignore the advice on here about not putting him on the birth certificate. Where there is no abuse, that is a terrible thing to do. The practical effect is to attempt to deny him parental responsibility. But he can get it anyway - he just has to apply to court. And make no mistake - courts understand the importance of both parents being involved, and WILL grant him parental responsibility. So, when you refuse to put him on the birth cert, you're just delaying the inevitable while getting your co-parenting relationship off on the worst possible footing. You're turning it into a war from day one, when it doesn't need to be.
So, make the commitment to working and parenting with him. You don't have to like him. But being a parent is about putting your child's needs before your own - and that means working with the dad, not against him.
Now, let's deal with his family. I've no idea how much of their behaviour is because they see you trying to shut their son / brother out of his child's life, and feel the need to weigh in and protect him. And how much is because they're arseholes. Could well be a combination of both! If his sister is sending you abusive texts, then clearly that is unacceptable and has to stop.
I would suggest that, once you've spoken with your ex and got things onto a better footing, you both sit down with both families and talk about how you are going to work together to raise this child. You will both need to be clear, together, that you need and expect support from both families on this. That means they need to follow your lead, and stop the hostility. He needs to be clear with his family, that he won't tolerate abusive texts etc. And you need to show them that you are genuinely committed to having your ex involved in the baby's life.
All of that will require you both to grow up a lot, and to put your own hurt feelings aside in the best interests of your child. Now is your chance to do that - before this all descends into court battles and open warfare. It's the right thing to do for your child, so I sincerely hope that you can both find the maturity to do that.
Good luck.