My boyfriend's ex is crazy

Well I have kind of a situation going on. I am 18 years old and I am currently in a relationship with a 27 year old. We met in July of last year. We starting messing around until November of that same year. He had just broken up with his ex girlfriend a month before I met him (June). I'm pretty sure I was his rebound girl or something like that  but I didn't mind cause I also had also just broken up with my now ex boyfriend. We kept going out on dates and messing around until one day (late January) he broke down crying and told me that his ex had called him and told her she was eight months pregnant. We were both very drunk that night and decided to talk about it the next day. The next day we talked about it and he told me that he would understand if I did not want to see him anymore and that if I did want to keep seeing him and that I could back down anytime I wanted. I decided to not stop seeing him since I was not his girlfriend and we did not have any kind of serious relationship. I know any other girl would've freaked out and asked to not see him anymore. Nothing really changed between us. In fact we started going out more frequently. She moved in with him and his roommates for the rest of her pregnancy. I didn't mind since he said she wanted him to be part of her pregnancy and I didn't want to deny him of that experience.She stayed for a month after the baby was born and then left to live with her mother. I was there for him for his baby's birth and was there with him when he once again broke down about him not seeing his daughter anymore.