I had this exact situation with our second child. For us, we weren't in the best place in our relationship (in fact, I'm 95% sure we had a HUGE fight the night we conceived DS #2...it was makeup sex, but probably not the most loving it could have been!) When I found out I was pregnant my DH got really upset. Our 1st son was a wonderful baby, he was 2.5 yrs old and we'd had a pretty good time being parents...so it totally threw me for a loop for him to be so upset about another baby! He was determined to terminate the pregnancy. I was adamant against terminating. It went so far as me "giving in" and we went for an appt. at Planned Parenthood to "discuss options". I spoke to the counselor privately first and I told her flat out that I had absolutely no intentions of terminating the pregnancy, but I was also worried about losing my husband over this (it was THAT bad of a situation..he was being completely irrational). She was fabulous and when DH came in we all chatted about his concerns and my concerns and she helped us sort through all of our feelings. After he got all of his feelings off his chest, he realized he had been ridiculous and he was just as responsible for the pregnancy as I was and we decided to continue with the pregnancy.
Unfortunately, the rest of the pregnancy was kind of tainted by that whole experience and it was a very sad time for a few months. We ended up in counseling again, and by around month 7-7.5 he started REALLY coming around finally. I leaned heavily on my midwives for support and our counselor as well (she specialized in issues around pregnancy/birth, so was perfect for our situation).
At the end of the day, he was my sole support besides the midwives during my labor and birth. He was incredibly loving and caught the baby in his own hands. And can I just say, DS #2 is HIS baby now They are SO SO close and just have this incredible relationship. DS #2 had the worst colic ever and DH would spend hours every night holding him and loving on him, and DH would literally get tears in eyes and told me over and over and OVER again how sorry he was that he ever thought of giving up on that little guy. It was, and continues to be, a very redemptive experience for all of us. We have a much stronger marriage now and I really think it has a lot to do with us going through that trying time and working together to get through it.
I'm sorry that was so long, but I just wanted to share my experience and hopefully give you some hope! I would say, as hard as it is right now, try not to harden yourself against what he's saying. *Definitely* stick to your guns if you don't want to terminate, but be willing to hear him out and let all the bad feelings be aired out. Find a 3rd party to speak to, whether its a counselor/religious person/whatever, who can moderate your discussions and help give another perspective. I think he'll come around, especially if he is normally a loving father. That was the thing with my DH, he's an excellent father and ADORES children...heck, we even decided to go for #3! Let me know if you ever want to chat more about this!