been there, done that. Sounds like Dante 3 years ago. He ended up thinner than on the track. Tried everything that you did. Weird proteins, no grains, etc. He seemed to be sensitive to grain and preservatives. As a last, and I mean last, ditch effort, I did the switch to raw. It was either that or watch him die. Even my vet told me that at that point, I had nothing to lose. We did the switch and it worked. However, he still gets loose at times and ground meat with fat, chicken skin, etc., sends him puddling. Rich organ meat loosens him up. However, I follow the guidelines in Switching to Raw and after a couple weeks, got the hang of it and it is almost second nature. Now, my two greys, whippet and 2 of the cats eat raw! You can also buy pre-made raw, but that has more fat than my own mix does. I do feel your pain, frustration and heartache. Good luck and keep us posted!
\Cindy