I have a
Koi with a very similar problem. I bought the fish a few weeks ago and she was becoming VERY thin. She was having a hard time eatting at the surface. I sometimes wonder if she has a problem with her vision too because sometimes all of the fish are on one side of the tank, and she's on the other. But she does swim over towards me so I have to assume she see's me or my shadow at the very least.
Anyway, I contacted the person that I bought the fish from and he said that often times because of interbreeding (
Koi's in particular it seems) have strange issues with eatting and their mouths. I started feeding the
Koi by hand so that I could get food into her. It started with her coming to the surface and I would practically put the flake into her mouth, I also tried micro pellets and she would gulp at them on the surface. the micro pellets floated longer and spread out better. Then I had a nitrite spike and I think it was from overfeeding (trying to get food into her). Now I find larger flakes and I hold them inbetween my fingers and she comes up and takes them from my hand. She's started putting weight back on, and looks great. But I can see I'm going to have to do this forever with her. I tell myself that she and I have "bonded".
I just wanted to let you know there is hope, don't give up on him/her just yet.