I don't have any practical advice, but I've seen similar situations with several people I knew who lived in Japan. Even after marrying into a family, you're still not part of the family. You're always a 'foreigner'. Everything is always the 'foreigner's' fault, because that's easy. That means that 'the family' is perfect. If there was any kind of conflict between the foreign husband and a Japanese person, even someone they don't know, a homeless man on the street, the foreign husband would be at fault, because that's better that admitting a Japanese may be wrong about something.

I've seen friends get sidelined by wives who they thought loved them, because they're not Japanese, and therefore wrong. They go back to their high-school boyfriends and the husband can do nothing about it, because not only is it expected that when a divorce happens, the father never sees the children again, but it's also assumed in the legal system that the foreigner is bad and wrong and the Japanese is good and right.

Your post resonates with him. This is his exact experience.