I take it this was an unplanned pregnancy? Were you doing anything to prevent it?
I would be terribly upset, but if it were an unplanned pregnancy, I would understand that he has a lot of thoughts and worries going through his head and that he might need some time to come around. Men can feel unprepared for pregnancies in many different ways, especially if they were not expecting it. He may not feel ready emotional, financially, or just not be in the place in his life where he wants children yet (or another child).
I would give him some time to think it over and let him know you're there if he wants to talk. If you are in a committed and loving relationship, he will work through it and eventually come around to the idea. Good luck!