I do understand how our children are our first priority, however my take on the ideal of a romantic relationship as unable to display priority due to kids is that our children are our responsibility but a romantic love is like night and day, in a way that a partner doesn't truly become second fiddle, so to speak. There's a balance. Children aren't our confidantes nor the one to rest our heads upon their shoulders for comforting. Children aren't our comforters.
As to the dynamics that go into a parent that would create such dependence is complicated. I've know some that do this. And sadly, there's such twisted logic that to even tell someone that it's not proper is like talking to a brick wall. My late stepdad had this dynamic, almost. She did live with us after a major storm. She's the same woman that told me uteruses fall out when women play sports. I learned it was better to bite my tongue and save it for unloading to my grandmother. He was her oldest son, and after his dad passed, that dependency between them grew. She had had a not so lovely childhood. Wound up stranded as a single mom of two daughters after her first husband walked out. My stepdads father took to her and her daughters in and married her and they had a family together.
So I'd probably look at your boyfriend's mother's history for an idea of what brings this dynamic into existence.
Your boyfriend could feel torn by honor to them and self sacrifice.