How should I talk to my fiance about his eating disorder? I'm so scared for him?!?

For now, tell him constantly how good he looks. Or that you love his body. Just love him a lot. Kiss him. Hug him. Continue to talk with his sister and if after a week or two he is still stressing over body image, then bring up the topic with him. Try casually first, saying something like, "You haven't been eating as much," or, "You're working out a lot lately." Talk to him about it without stressing him out and then try to get help if you and his sister honestly think he needs it.