Going back in after pulling out?

My boyfriend and I are extremely happy together. He is moving in a few days while I am staying here to finish my last two sememsters of college. Then I will move up with him. We had un-protected sex and he pulled out, came, and then went back in afterwards. I realize you could obviously get pregnant from this but I'm wondering the percentage or how high of a chance it would be. Truthfully, we both would be very happy with a baby, however we both would like to wait. Any comments??