Its possible its an excuse and a really disgusting manipulation but also maybe its not? Idk grief is a weird thing. How quickly did he get into a relationship with you after his ex? Does he show his child affection? Is it possible he was trying to avoid the reality of his situation before by recreating everything and having it turn out "right" this time but it hit him that this baby is not that baby... Which would really fucking suck :< he could have just self consiously or not but with therapy he could overcome this. Either way he needs to go counseling. If your just tired off this then yeah you can leave its always an option. I say sit on it for awhile set a date for a your decision and just see where you land maybe consult a therapist yourself on the matter or just ask him more about it and listen suspend your judgment until the day you set to decide comes up.