My 2 yr old Great Pyrenees doesn't want to eat his regular dry food. Have tried changing to food for sensitive stomach, mixed with boiled hamburger, chicken, rice, pumpkin puree. He's not interested in treats either. He had a checkup recently, treated for whipworm, and is clear. The vet said he will eat when he wants to, but it is totally a change in his behavior. Advice please!