NANO is almost a year old. Friends of ours gave him to us because he was nauty and always jumping over fences, but since he stayed with us he still tries to get out but that problem has beeb fixed the only problem left is the food. I dont want to give him everytime something else to eat because they are going to get used to not eat their pellets because he know if he doesn't eat i will give him something better. MY other two dogs eat the same pellets for over two years now and they still love it, sometimes they all get a bone after a braai but not everyday.
First, check his teeth. The vet should have done that but if not check yourself. One may be absessed or something else may be wrong. You could also go to a different vet and get a second opinion. Did the vet think he was too thin?
I personally do not change my dogs food, but I will add bonus treats to it. My dog loves green peppers, sour cream, gravy etc. If you didn't want to add a topper to his current food, you could try just adding hot water. That will give it some flavor.
You could also try feeding him in his crate. Maybe he feels threatened by the other dogs.