OP, 5 years ago, I could've pretty much written your post word for word.
I'd say give him time.
My DH was booked to have a vasectomy which had to be rescheduled - in the intervening time between the first and second appointment, I got pregnant - it was a complete surprise.
We considered a termination, well, In my heart I never really did, but I went through the motions with him as he wanted to consider all the options. I couldn't go through with a termination.
We barley spoke for about 2 weeks after that - about anything. My DH had a vasectomy when I was 12 weeks pregnant (about a week before the dating scan).
I won't lie to you, it was hard. I wondered if we would come through it, but as time went on, he spoke more and more about the baby, but he was still very guarded. I think it's hard for men to feel emotional at this stage as nothing has really changed for them, whereas you have the baby growing inside you with all the hormonal and physical changes that that brings. Once the baby was born, he was fully on board (he has always been a very hands on Dad). It helped that DC4 was a boy after 3 girls.
Again, I won't sugar coat it for you, but the first 2 years were bloody tough, I wondered what on earth we had done. I suffered terribly with PND, and sleep deprivation. It put a huge strain on our finances and our relationship.
However, our DS is now 4, and life is much easier, our relationship has recovered and I'd even go so far as to say it's probably better now. I love having 4DC, but there is a part of me that hankers after what life would've been like with three; we would've had more money, more space, more time, my career would've recovered more quickly, I'd have more individual time with each DC. However, I love all of my DC with all my heart, and couldn't imagine life without any of them.
It's not a bed of roses, but it's better that I though it would be, back in those first dark days.
It sounds like your DH will get on board, his comments about getting excited about another sqooshie speak volumes to me. My DH never said that, although he comes into his element once they are over the baby stage.
I feel for you, it's a long and difficult road, but if it's what you want, I think it'll be ok eventually.
Take care.