My [26F] Fiance [31M] just told me that he's not attracted to me but it's not something I can fix.

Update: I wasn't expecting the huge show of emotional support here. Thank you everyone, I am reading every single comment. I just wanted to come back and update my situation today. I actually did decide to break up with him today. Most of the stuff in our apartment is mine, so he just packed his things and left. There was no fight, he didn't even say anything about wanting to stay or work it out. He packed his things quickly and quietly, gave me his key and told me I could keep his bike and the stuff in our storage. I came to the hard realization that this just can't for for us anymore.
So we have been together for 6 years and engaged for 2. I suffer from a few autoimmune conditions and unfortunately I cannot put on weight. It started to get worse the last 3 years but I'm medicating and managing the best I can.
It sounds wonderful, I know, but it's not. I'm really bony and he always tells me I'm uncomfortable to cuddle so I end up going to the couch. I have a hard time keeping a lot of food down so I use a lot of meal shakes. I work closely with my doctor and do my best. I exercise lightly to stay strong. I just can't put weight on. I know it's not attractive. I bundle. I put pillows over myself for him. It's honestly work just to stay at a borderline safe weight but I try. He recently told me that he just prefers curves. I will never be voluptuous, he knows that. He only wants to watch porn with curvy girls instead of be with me. He wants me to just eat more. But the weight won't stick. He said that he sometimes just pictures a different person from the neck down.
How do I explain to him in a way he will understand that I can't change my body like that? Is there something I can do to help satisfy him? I'm at a loss.
TL;DR Fiance isn't happy with my body but I can't change.


Source.