ANSWER:
Marriages that are not based on Islam often face such problems and solving them is quite difficult as there is no focal point to refer to.
Talk to him when he is in a good mood and explain to him that the matters between the husband and wife should not be disclosed with anybody. He is sinful and committing a major sin as this is backbiting!
If you live with them, ask him for a separate accommodation as this is your right in Islam and then you won’t care what they think of you.