My dad was like this growing up. Then as he aged he had to change his eating habits. He was so thin still but began having blood pressure,cholestrol issues.
Anyhow I feel you can't change others. In terms of meals could you make the same main meal but for your fiance have some extras that are quick like rolls, corn on cob,larger portions? He can add juice or milk to meals,have double portions as well.
If he won't eat a main meal the same can you each take turns cooking? Or maybe make quick items such as if you have chicken you can put a steak in for him?