My boyfriend Ex is pregnant with his baby should i stay?

Ok so I met this guy at a new job I started and we hit it off immediately and we got together he was in a 4 and a half year relationship and he claimed that he was unhappy for a long time but did no know how to leave and I understood him. He told me that I give him life that he has never felt before that I make his day brighter everyday. One day he was at work and I was sleeping and he texted me and told me that he had sumthin and I already knew it was sumthin bad so he called me and told me that she is pregnant and I really didn’t know what to do he said he doesn’t want to be with her, and she already had two kids with two different baby daddy’s, and she is only keeping the kid because he doesn’t want to be with him ita just a lot of drama she has the mind of a 16 yr old (she is 27). The bottom line is I really want to be with this guy but I constantly think about having to deal with her she tries to throw it in my face and I want to slap her there is no tlkin to her she won’t comprenhend I feel like boundaries need to be set but everytime I think of her carrying his kid it makes me sick and I know I take it out on him but like wat about the future I don’t know what to do seriously I need advice.