Adding another baby to blended family

My SO and I have 5 children. He has 3 and I have 2. We've always talked about one more and he loved the idea. He had been talking about it recently and then all of a sudden says he is done. He doesn't want anymore and is completely done. He wants me to get fixed. I am hurt because it is something we had talked about and agreed on and now he has changed his mind. Does anyone know what I could say to him or how to talk to him? Every time I try to talk to him he says he doesn't know what to say to me or he'll just say something that'll hurt me. I just don't know what to say to him and I really want another baby.