Waitaminute- you and he are having a CHILD together, in THREE months, and he dumped you because he THINKS that will help his other relationship??
I'm sorry, this is probably going to come out harsher than I intend, but I'm angry. It angers me when someone uses poly as an excuse to lead with their dick/vagina and ignore the other responsibilities they already have. I'm not saying people need to stay together for the sake of the kids if the relationship is no longer working, but he broke up with his PREGNANT GIRLFRIEND because he was GUESSING that it would help his other relationship. That's beyond immature.
1) He needs to learn to communicate. If he discussed her mixed signals, distance, and lack of commitment with her and she SAID, "I can't commit to you while you're in a relationship with her, it's one or the other." then maybe he has a leg to stand on, if he really truly thought about which relationship was better for him in the long run and recognized the potential consequences of his actions.
2) He better be ready to man the hell up. Whether you and he are in a sexual relationship or not, you absolutely are in a relationship as co-parents. He had better still be prepared to spend a LOT of time with you (and you and the baby after it's born) and to put HIS CHILD first for a while. Babies are hard. They take time and attention and they can strain even the best of relationships, which you two are not starting with now.
Being pregnant and being newly poly is a LOT of change all at once. Even if you were all the best communicators in the world (which doesn't sound like the case) it would have been rough. I've been through two pregnancies and damaged a significant relationship each time, though thankfully each has since been repaired. I hope you two are able to be peaceful with each other (you do NOT need this extra stress) and I hope he's able to take a step back and reorganize his priorities.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack