SO dropped a bomb

So I've been dating my SO for almost six months and he dropped a major bomb on our relationship yesterday. He has two children from a previous marriage and I have one from mine. Yesterday he tells me that the girl he dated before me is pregnant and claiming it is his. He is uncertain that it is but after DNA testing will step up and be a father if it's his. However she is due in a month and I'm just not sure how I feel about the whole situation. I love this man and I am willing to work with him through this but what would be my role in these kids lives? I want to have a child with him one day but four kids just already seems like a heavy load. I'm torn with jealousy because of this and he is afraid he is hurting me constantly because of what happens with his ex wife and now this. My daughters dad is not in the picture so he would end up being that father figure for her, but I would just be the step mom. I'm not sure how to mother a child from a distance. Any advice would be appreciated. Just trying to figure out how to handle all of my emotions.