I'm so sorry you are hurting right now. My husband never changed his mind- I was the one who changed my mind! Our circumstances are probably much different- I have two kids from a previous relationship. My husband knew going into our marriage I was dead set against more children. I told him on our first date. I told him a few weeks before we were getting married. We fought about it weeks after our wedding. He really wanted one of his own, "our own". He quieted down and left it alone, maybe bringing it up every few months. I softened after time went on. As my kids got older (they are now 10 and 8) I feel like our family isn't complete. The toddler ages were overwhelming, esp having 2 in diapers at once, etc. My advice is just be patient. I know it will be hard.
Have some dialogue as to the real deep rooted issues behind why he feels your family is complete. Discuss what you feel you are missing. Does he enjoy being a parent? Are finances secure? Is he depressed? Saying "our family is complete" seems very vague, however it is his answer right now. How old are the little ones? If they are young, toddler age, it might be overwhelming. Maybe wait a few years and revisit. It may not happen on your timeline/timeframe, or if ever. Overall, what changed my mind is waiting some years, enjoying watching my kids grow and become more independent. Now I have older kids who can "help" if a little sibling ever comes along. I would find out a little bit more- respect his reasons, understand his point of view. You may disagree, but give him time and space, talk about it in a few months. There is a good book that helped me called Creating Your Perfect Family Size. It quieted some of my anxieties I had and still have. Good luck!