My fiance is starving himself, and I'm really scared for him... How should I talk to him about it?

Wow, this is a tough situation. My advice would be to tell him all the things you mentioned about how much you love him, you can't live without him etc etc. Tell him sincerely and tell him often. My guess is he is going to have a really hard time believing you but don't give up. Tell him how worried you are about him and do your best to convince him to seek professional help.