Okay, I went through this with Ronin... that boy would NOT eat to save his life he'd go days without food and not miss a beat. he started to look like a walking skeleton.
I tried EVERYTHING... all types of dry kibble, wet food, tripe, cottage cheese, yogurt, supplements, satin balls, nutrical, raw meat, vitamin B, ground up dog food with water and gravy added, even macaroni and cheese... NOTHING worked.
Literally. Nothing worked, and it worked well. Once I stopped coddling him and worrying about it, and decided that if he wanted to be a skinny dog, well gosh darn he could be one if that's what he really wanted....
.. he started eating. Now, he needs nothing to convince him to eat - I scoop out his kibble and plop it in his bowl, and he eats it.
Give your poor dog a rest and stop worrying about his food. If the vet hasn't been able to find anything wrong with him, then don't worry about it - stop caring, and he'll probably start eating.