You know, I had an eating disorder, Lithium worked fine for me until recently when it made my weight shoot up, and i am now on topamax to get it back down. I understan a bit of what you are going through, and I think it is alright to say to your guy that this is not a topic you want to keep having brought up. Perhaps it is better to put it in terms of "i need to deal with this myself without others getting too involved."
A lot of men think hopping in and taking over is supportive. it's not, but that's how they were raised to think of things. My mom does the same, and it's very difficult to manage her, because with her, itis all or nothing; she'll even say "Well, then, I just won't care atall," if asked to just back down a little. Maybe his pity party is a way of trying to get some help in how to manage this. See, for a guy, this IS a bawl out and it IS about him; the WORLD is about them! I would take it a bit calmer later, but let him stew a moment. It's not his concern, really. Your body image is YOUR body image and NOT HIS pet project. There is a difference.
HUGGGGGSSSSSS and good luck. He sounds like he loves you but just needs to stop navel gazing. Keep up the work and see what you can find to help take your own focus outwards.