My fiance's ex wife wants him back?

My fiance and I got engaged on Valentines day and we are expecting our first child in August. I know this will sound a little silly but I am a pretty intuitive person and i sensed something was wrong for the past couple of days. He finally admitted his ex wife has been texting him and wants him back. They have three children together and she is saying the children would be happier. I am glad he told me and he admitted he considered it that the part that hurt. I know they have tried before and failed becuse she cheated on him etc...so I know it would not work at least I seriously doubt it. I love him and the irrational side of me says leave, I make decent money and can afford it but the rational side of me thinks I am jumping the gun here. How do I deal with the situation. She is going through a divorce and does not want to be alone and I feel that she also can not stand to see him happy. Advice please.