It sounds like he may have a touch of pancreatitis. Pancreatitis is an inflammation of the pancreas. The pancreas is a gland that produces and releases enzymes into the digestive track that aid in the digestion of food. When these same enzymes are activated while they are still in the pancreas, inflammation occurs and the enzyme start "digesting" the pancreas itself. Some common causes of pancreatitis are eating a lot of very fatty foods, obesity, steroid use and other diseases and conditions.
Symptoms of pancreatitis vary but include vomiting, diarrhea, anorexia, weakness or collapse, dehydration, shock, fever, depression, and upper abdominal pain. A dog may indicate abdominal pain by acting restless, panting, crying or wincing when picked up, shaking, standing with an arched back, or lying with the front end down and the rear end elevated You can read about it in detail here:
With a touch of pancreatitis, he may not have all the symptoms. Normal treatment would be to withhold food for 12-24 hours initially. Your vet would use an IV and monitor him. If he isn't eating much, you can put a drop of karo or pancake syrup on his tongue to keep him from becoming hypoglycemic. After that offer small portions of bland food such as boiled chicken and rice to see how the dog's system does with that. If your dog does fine, you can slowly reintroduce your dog's regular food into the chicken and rice mixture until it is all regular food. Keep portions very small.
Pancreatitis can be serious and is painful, so we do recommend you have them seen by your vet. If you can get him eating then he should be ok.
I hope you find this information helpful.