Well, there are lot's of younger dads out there! Any chance that it's not the "young" part, so much as the "dad" part? I think a lot of guys kind of panic when they realize that they are definitely committed to the parenting thing! The baby IS COMING, and they are going to be dads! So it could be that you will not be able to "reason" with him about this feeling, just reassure him that it will be OK, and he will be a great dad.
That being said, there are so many wonderful things about being younger parents that you can pass along to him. Energy level is one major issue. Older parents, I think, do not have the energy or stamina of younger ones.
And children bring so much richness to your life - why put that off? A baby brings some of the biggest challenges and biggest joys of your life. Parenting causes "soul growth", if you know what I mean. You will be given opportunities to laugh at things you never would have otherwise, and to notice things the way a child does, and to see things the way that they do. You will have a little person who thinks that you make the world go round, and that you can solve any problem that they come to you with. Why would you want to wait for that? They will of course, make huge demands on you too, but this also will teach you patience, compassion, sympathy, and self denial. These are character traits that make you a much better person.
Older parents are often fairly set into a routine, and children can be a much bigger disruption then as well. Younger parents can often be more flexible, which is necessary!
Of course, there are many older parents out there who are doing a wonderful job of parenting, and age certainly doesn't determine whether you will be a good parent or not, so I hope that no older parents reading this will be offended, but over all, I would definitely recommend becoming parents at a younger age, rather than older.
Maybe it would help your DH, Ryn, to have some discussions with other younger dads? If all he is seeing is 40ish parents, no wonder he feels out of place! A a few new friends might give him the support and encouragement that he needs.
Anyways, I hope that he comes around soon, and you can feel like both of you are eagerly anticipating this baby together!
By the way, my DH was 21 when our first was born, my BIL was 20 when his DD was born, and all of our close friends have had their first kids in their early 20s. Our parents also, both got married around 20, and had their first a year or two later. So for us, it's pretty normal and doesn't seem young at all. So I really think that having some friends in a similar life situation to yours might be a real help to your DH.
HTH!
All the best.