Get Your Dog Care Questions Answered by Experts
I would call your vet and take him in for a checkup. Depending upon his age, he may need special medicines to treat him for constipation, or perhaps the food you are feeding is not agreeing with him. He could also have parasites, such as roundworms.
You need to have him seen, as an animal can become dehydrated quickly if he is not eating and he could also have a serious viral or bacterial infection. He may need medications from your vet.
Please take him as soon as possible.
God bless and good luck!