I think because we are the ones that go through the pregnancy and birth and all of the hormones, physical changes and pain that come with it, we forget about the impact that the whole thing has on our DH's.
They love us to bits (assuming that it is a good relationship) and it must be horrible seeing someone you love wholeheartedly go through the trauma of pregnancy and birth. We tend to worry about whether the baby is healthy and while the 'what if' is there about our own health, that is more of a background issue because generally thoughts of the baby consume us.
Our DH's on the other hand are faced with the fear that something will go wrong and they will lose the woman that they love. They don't have the same physical and emotional attachment to the baby that we have, so all the worry and stresses we have about the baby, they have about us.
It is very easy to say 'what's the problem? It's not like they have to go through pregnancy and birth' but having a baby is so much more than that and to call him a knob (as a few contributers to this thread have done) for not wanting to see the woman he loves go through it all again minimises his feelings and reduces his role to sperm donor which is very unfair.
OP - I'm glad you have been able to sit down and have a conversation about it. 5 months in really is very soon to be making the decision to have another. We always said that we wanted 3 kids, but after DD1 was born, I couldn't even begin to imagine having another until she was 2.5. DD2 was born just after DD1 turned 4 and is now 7 months old. We definitely still want (at least) 3, but the difference is this time, I'd start trying for number 3 now if finances would permit it. As it is we're going to have to wait a while.
While your DH is saying not ever, is reasoning behind it sounds, to me anyway, more like a I'm just not ready to go through it again yet because he is still recovering from nine months of constant worry. No one can say for certain whether he will change his mind, but in 12 months time when you have had your daughter for longer than the amount of time you were pregnant, the worry and stress of the pregnancy will seem like an age ago and I suspect his feelings will be very different. Right now, it's probably still very raw for him.
Enjoy your beautiful family and your DH who clearly adores you.