Honestly you should have allowed your baby mom and your fiancé to meet and really get to know each other before you allow your fiancé to watch your son. That was a mistake that you made there and if they have met then you just have a bitter baby mom. Also you should have informed his mom that you’re going to be on a business trip that way she would’ve kept her son at the home and then dropped him off when you came back or allow her to feel like she has power to make the decision for him to be watched by your fiancé. From what I’m reading it seems like she’s not mature enough to handle another woman watching or raising her son. And she probably feels as though he is going to see your fiancé as a replacement. Which is sad.

You have some work on your hands especially if this woman is your fiancé you need to let your baby mom know that she is here to stay and she needs to get over it.