SO says he wants baby, but pulls out ?

Hello ladies me and my SO have been been together a little over a year. The baby topic has came up many times since we are living together along with my SO 3 yr old son. We've agreed that since we are not using birth control , pregnancy would be okay if it did happen. But lately my SO has been pulling out. Any one else in the same boat ? It makes me feel very upset and sad. It's such a hard feeling to describe, especially when your ready to become a mom. How should I handle this situation?