Boyfriend has a son

So my boyfriends ex girlfriend had a baby and is claiming it is his. The timing lines up and we're all pretty certain it is his son. He claims it was a one night stand and was before we got together. I believe him and I want to be okay with it because it was a mistake he made before he knew me. However, my heart hurts every time I think about him being happy with a baby I'm struggling to give him. And I don't like the fact that I now have to share him with his baby mama. I'm not sure how to handle any of this, and I don't know what questions to ask myself or anything. I love him and i refuse to give him up. I would love if any of you lovely ladies had some sage words of wisdom for a new/soon to be stepmom. Or have any advice on how to be more okay with the present situation I have been plunged into. 
Edit: On top of everything, he is in the Army and only told me right before he left. I haven't seen him for the past 5 months. I'm trying to be strong for his sake, because I know having a son means a lot to him. I'm just struggling with the fact that I don't get to share that joy with him. She has been pretty civil with us so far, but that doesn't help with the fact that I still feel like I'm the one standing on the sidelines. If that makes sense.