You’re only 25. You yourself have a child from a prior relationship and are now pregnant with your fourth. No wonder you’re feeling overwhelmed and scared by this unexpected situation. But — if you love the man, you’ll find a way to love his son — just as he needs to open his heart to your child with your ex. The kids didn’t ask for this situation. They need to be known and loved by their parents. And they need to be accepted by whoever is in their parents’ life. That goes for all the adults in this picture.
The former girlfiend isn’t breaking up your family. You and your boyfriend can make it or break it. The two of you have the choice to make your family bigger and stronger by embracing all the kids or to tear it apart through your jealousy and his what-ifs. Your children need both of you to be mature, responsible and loving toward each other and towards all of them. Call your boyfriend and apologize for asking him to move out. Have a heart-to-heart conversation about how to be a team and how the two of you, together, can make his son welcome without threatening the family you have.
I wish you well.