Well, the only thing you can do is tell him the truth about how you feel. If you were concerned about this situation coming to pass did you not use birth control? I'm not just talking about the pill, but condoms? I am aware that BC fails (very very aware).
Tell him the truth. That this is surprise for you and you don't want him to feel trapped. Go from there. You have options to discuss with him. Adoption? Abortion? Parenting? Take the time to go through it all with him.
Yes, its overwhelming. I'm sorry you are feeling so side-swiped by it.