You have to be open, honest, and blunt with him. If he loves you truly, he'll understand where you're coming from. This is a life-changing event that will affect you, and if you are not ready, he needs to know. This is a two-way street, and it's not all about what he wants.
Maybe gently sit him down and tell him, or tell him with a friend or family member if you feel more comfortable doing so.
But keep in mind, if he truly loves you, he will understand that right now, you are not ready to bring a baby into the world.