Give him the reassurance that you will be together, through thick and thin, and that your family won't be split, that when something goes wrong in your relationship you will fix it together not throw it away and that you are so excited to be the mother of his child, i'm sure it is hard on him and makes him think of his other children but a little moral booster should do the trick:-) good luck and congrats
You might want to calmly explain to him that you are very excited and it's hard to stay that way when he continually has a negative attitude ..that you and her are two different people and you and the new baby shouldn't suffer from the turmoil with his ex..if you do bring it up, it may help to rehearse what you wanna say. Pregnancy hormones can take control when you're not prepared LOL. good luck!