My german shepard wont eat ?

My german shepard hunter is 6 months and weighs approximately 50 to 60 pounds. Hes not fat but all muscle. The last time i took him to the vet the doctor told me he was teething. Lately he wont eat. He has been picking at his food. I dont want to give him any cheap food. Ive tried many different types of dog food. Ive been feedin him "eukanuba". The only way he eats it is if i mix it with water or milk or egg. I dont know if its because his teeth are bothering him or what. Can someone please explain why he wont eat. Hes not sad or anything. Hes a happy and playful dog. Can someone also recommend any GOOD dog food for GSD