@Faith-and-Hope, there's the massive driver in NPD that any relationship is only valuable for as long as it's a pedestal for the person's ego. So sadly, it follows that multiple relationships are a bit characteristic. It sounds like his mother has been an ongoing "pedestal" for him, and probably his doctor who thought his lifestyle was such an improvement in fitness, but without you supporting his behaviour, it makes horrible sense that he would go looking for other female "pedestals" to fill in where you were supposedly "failing" him. Very supposedly. NPD people are very deeply damaged, and I so often have wished that our ex-friend could have seen the wasteland that she was turning her life into... but if they see any critcism as a withdrawal of ego support (their perception of "love"?) rather than the constructive warning it's meant to be, you get shut down and shut out.
I'd be watching your kids very carefully... they are another of the NPD pedestals, and yes, very vulnerable. Maybe D2 particularly so, because she would be easier for him to manipulate. 😞