Well, I hope things are a little better for you today Ryn. I know sometimes it can seem overwhelming when you don't feel like DH understands, or is on the same page as you. I'm afraid I don't have any wonderful suggestions for you, except to say that it really sounds like your DH is afraid of breaking the mold set by his parents and brother, and becoming nervous about being a father (I think that the age thing probably isn't really at the heart of it). He needs to be his own man, and make and take responsibility for his own decisions. But that is a personal change for him, and you can't make it for him, or say any "magic words" to make it happen. I have a feeling that it would help to do your best to make him feel like he CAN do it, and he will be a good father, (maybe even a better dad because he is younger!).
It feels backwards, I'm sure, because you're the pregnant one, and he's supposed to be supporting you, and making sure that you're OK, but hopefully soon he'll be OK, and you'll have encouragement and support for yourself again from him. I guess that's part of what marriage is right - give and take?
By the way, I think I remember you posting elsewhere that you attend church, but DH doesn't feel comfortable coming with. Is that right, or am I getting confused? Are there any young couples in your church with little kids? Maybe you could meet for tea sometime? Otherwise, the antenatal course is probably a great place to meet some couples in your situation.
Is there any chance that you guys could take a weekend away together and find away to (carefully and gently!) talk about your hopes, fears, concerns, etc?
Does your DH have any close men friends, or family? Does he talk to his dad, or his brother? Would they have encouragement to offer, or would they be more likely to tell him that he's right to be nervous, and he was foolish not to do what they did?
Anyways, those are the best ideas I can come up with, and like I said before, I really hope things sort themselves out for you soon, cause it's miserable to feel alone in any way in a marriage. It's not your fault if he feels this way, so don't feel guilty about it!
All the best!