How long has he not been eating???
My GSD will do that some time, my first one wouldn't eat to the point that he would throw up bile. My vet said that some dogs are just picky eaters, and some still eat like a wild dog/wolf; they won't eat for days then they gorge themselves if you let them. My first GSD did that, I started to just leave the food out for him and he would eat when he was ready. I know for some it's hard to do that; I wouldn't be able to do it now that's for sure, but can you leave his food out? Does the vet seem to be worried about the weight loss? Oh, some dogs get funny about eating when the weather changes too.