Well, the problem is how your finance is having to deal with the influence your son's real mother is having on him. She is right in that he is not her son where she has the ability to prevent him from being exposed to the negative things that may be going on in your ex wife's home that he is subjected to. In turn this is a challenge for your son because no matter where he goes, either home, he probably feels like he doesn't quite fit in.

I agree with the other posters in that your son could benefit from seeing a therapist who can help him with how he struggles with this challenge. Also it would benefit both you and your fiance to talk to a family therapist who can provide you with some helpful tools the two of you could use to handle this challenge. After all, you are the adults in this picture and your son is depending on YOU being able to handle this challenge.