Okay, this is a different scenario, because of your closeness to your fiance.
Go into it understanding that every person has different reasons their body is the way their body is. A person can make healthy food choices and get exercise and still be fat. Because that’s their body, and that’s okay. Check out the Health At Every Size movement for info and resources on this. Like, my doctor is happy with my resting heart rate and my blood sugar is totally fine. I make it a point to get exercise. I’m always going to be fat, and that’s okay.
Without knowing your fiance’s situation, there’s no way to know if he’s living a fairly sedentary life or if he’s already active. If you’re worried about your fiance’s longevity, try to do things together. You can make health a priority without ever mentioning his weight. You can take a dance class together. You can take up hiking and set goals of hiking to a certain place. You can do some distance biking. You can take a cooking class together, or find recipes you want to try and make cooking a joyful, couple-bonding experience.
The important thing is to focus on health and not size, and to understand that he might actually BE healthy at his weight.