It's funny how our emotions can work. You want something so bad and then you get scared. Take getting married for example. People get so excited for it and then on the day of the wedding, or a few days before, start freaking out! When getting married was what they had been excited for all along! And other people aren't affected at all.
He is probably just feeling a huge mix of emotions. I know my partner has been going through a lot of mixed emotions. I think time will bring your partner around, and especially as you get to see the different ultrasounds, as they start looking more and more like a baby, when you find out the sex, when you hear the heartbeat. I think guys tend to get more excited when they have tangible things. We women know the baby is inside us. We can feel the changes in us, but they are just a bystander. Their job for pregnancy was done at conception, you know? My partner has said before that it feels weird because nothing is changing for him right now, it almost feels unreal.