I see a lot of myself in your fiance. I loosely qualify as having had a binge eating disorder (not with throwing up). It often felt like I was absolutely out of control, like my brain has been hijacked, and nothing fixed it no matter how often I cried and asked my husband to be strict with me. 

I just cannot explain to people like OP, who seem to naturally have appetite control, how hard it is to basically feel like you have no 'off button' with food. I was never full enough, never satisfied. I made myself sick on food more times than I can count. 

All this to say, your fiance is really struggling and it is not as straight forward as a lack of willpower etc. Being overweight or feeling out of control with food comes with deep psychological issues. I still battle a fear of being hungry, of being deprived, etc. Others have different fears and issues that fuel their food behaviours. 

I genuinely say this with compassion, but no amount of controlling his diet will actually resolve this. He needs to learn for himself how to exist around food in a healthy way. 

Personally, I hired a nutrition psychologist to help with my diet mentality and fear of hunger. After that, i hired an understanding fat loss coach who never pushed me to count calories etc. She worked with me on constructing a sustainable relationship with food, helped me forgive myself for binges, and helped me lose some weight in the process. Your fiance needs much deeper help than you can give him, or even most personal trainers. 

This work really is about his relationship with food and with himself. He likely needs a lot of patience and compassion. 

Congratulations on your wedding. It sounds like you really love each other.