Hi C.,
Congrats on your pregnancy! I was pretty young when I got pregnant for the first time (twenty), but my husband was even younger. He was seventeen when his first girlfriend got pregnant with our oldest daughter, eighteen when she was born; our second daughter was born (to a new girlfriend) when he was twenty-one.
When I met him he was twenty-three, with daughters ages five and two, and acting like Peter Pan. Although he was a father of two, he was still acting like a young bachelor--his mom and I took care of the girls whenever they were with him. He undeniably loved and cherished our girls, but he was not acting like the mature, responsible father he should be (I second the mom who wrote that man-children come in all ages). It was especially hard on me once I came to fully love our daughters and consider them as my own; there were times when I would be taking care of them and he would be off with his friends, having a good ol' time, while our kids were asking me where he was and when he was coming back.
Today he is the responsible father he should've been from the get-go: he has sole custody of one daughter and joint custody of the other, he works, cooks, cleans, spends time with all four of the kids and just enjoys being a dad. He really started acting this way full-time in 2001 or so (and it still took a few more years for him to fully develop as a father); for him, the initial journey from man-child to dad took six years.
Is your boyfriend scared? Sure! Will he turn out have the same difficulty with growing up as my husband did? Who knows! He could be just "getting it all out of his system" now, like the other moms mentioned. He could surprise you both and be the best father and partner to you in the whole world once the baby is born. No matter what happens, the pregnancy is definitely going to keep changing your relationship, the baby even more so. I hope it changes it for the better, but you seem to be aware that there's no guarantee of that.
I definitely agree with the moms who said to keep your family around for support. Family is invaluable resource when you have kids of any age. It's important to keep your boyfriend's family involved with the pregnancy and the baby too. Make sure you let them know that no matter what happens with you and your boyfriend you'll always make sure they're a part of your baby's life (I tried to go in the other direction once and cut my husband's family out for good--trust me, it's not good and it's not worth it). I don't know how comfortable you are with them, but if the answer is not very, I'd say you better get comfortable if you can. Nothing is worse than having wars with your in-laws over your relationship with your boyfriend and/or your children. I've been there, and it is AWFUL. It hurts everyone involved, especially the children.
Good luck, C., and just try to keep your head up. No matter what your boyfriend does or where your relationship goes, just remember that the baby is what's important now. If you two can always manage to put the baby as your highest priority you'll always be on the right path. I'm here if you'd like to or you need to talk privately, and I imagine many other moms are too :)
--M.