This fellow is in no way ready to be engaged to you. My guess is that having a newborn daughter is making him rethink a lot of things. It should. He has brought a child into the world that he is responsible for. If there is any possibility that he can make a family for his little girl with his ex, he should at least give it a try. His daughter deserves it. If he and his ex can’t manage it, he will then need to work with his ex to figure out how to be a father even if he can’t be a husband to his daughter’s mother. This is difficult, difficult stuff. It shouldn’t be rushed.
Please remember that the way he treats this situation, and this child, is a forecast of how he will treat you and any children you have together. I think you should take a big step back and give him the room to do his personal work in resolving his prior relationship one way or another. If he doesn’t, it will get played out in negative ways in his relationship with you.
At 18, you have plenty of time. I urge you to slow down and learn more about yourself before committing to a marriage. Get some education. Develop your career. Find friends who share your adult interests. You are going to be an adult and parent for a long, long time. Don’t miss out on the chance to be young and single. With no responsibilities other than to yourself, you can explore lots of new opportunities and experiences.
I wish you well.