This does sound like a lot for your son to take in, having just gotten used to the idea of his dad having a new girlfriend. The polyamorous lifestyle isn't easy for even some adults to understand. Now, if you could clarify - is he actually having sex with this other man, or is he only in the relationship with the woman, but her boyfriend is also there? I have a friend who is Poly and she kept her husband and her boyfriend very separate. At one point, her husband's girlfriend was living with them, but I don't think she and my friend were a couple. It gets confusing at times. Will this be an overnight visit? Or can he just go meet his dad for lunch and meet the new girlfriend there and avoid the whole who-is-sleeping-with-whom scenario? Your son is also getting to an age where he can decide whether or not he wants to see his dad. If he doesn't have to be there overnight, I think a daytime, meet the girlfriend in a neutral place sort of visit might be best this time.

As far as what to tell your son - it all depends on the extent of the relationship between the two men. If the two men aren't in a relationship of their own, then you could just tell him that his dad's girlfriend has another boyfriend and that they are all living together. If the two men do have a relationship of their own, that will be a tricky one, given your son's trauma history. I wish I had words of wisdom for that scenario, but I'm afraid I don't.

And yes, I would be concerned about the financial arrangements as well. Are you getting any child support from him?