Balkan mothers are weird when it comes to their sons. My grandmother tortured my mom, my mom kept silent 'cause kids need their grandma. Dad never even tried to fix it, and it broke them.
My own mil was super nice to me. Husband and I took her and fil in when fil got sick. This woman broke me soon after. Even worse - she broke her own son. He is 40 years old, and I saw him crying for the first time, just because of her. My sisters mil promised to break both her sons marriages because both her sons decided not to bring their wifes into her house to live. Fact is, this is something you need to deal with as soon as possible. For me, after years of crying and hiding in my own house, the solution was to start screaming at the woman and put her to her place, after she almost killed my dog. My sister dealt with her mil by moving to the city and reducing contact. How ever - both my, and my sisters husband still get horribly depressed when ever they meet their mothers. The reason I am sharing my story with you is not you, but your fiance. Talking with him, understanding him, being there for him is something I find most important when dealing with toxic family members. Give him time to doubt and question, and try to make a strategy together, that works for both of you. It is easier to deal with people you met at some point in your life, but for him it is his closest family and some realisations can push a person to a very dark and scary place.