We cannot diagnose your dog over the internet. He needs to be seen by a vet and SOON. Whenever a dog stops eating there is something very wrong. An animal's instinct is to hide any signs of illness or pain because in the wild, to be sick or lame is to be prey for another animal. So they will act normally and eat normally until they just can't stand it any more. That's the problem, so much of the time, that by the time they show us they are sick, they are VERY sick and can be in a life or death situation. Please take him to the vet immediately and let us know what the vet said.
Ghilly
What does your Vet say is the problem?