Thanks for the additional information.
Since he has had multiple puppy shots, then parvovirus shouldn't be an issue which is good news. However, parasites and protozoan infections can lead to nausea. If a dog has nausea they might not want to eat or drink. Dogs who have had a meal with high fat content can also develop nausea related to the onset of pancreatitis and will often stop eating and drinking as well to help relieve the discomfort. Constipation can often cause a loss of appetite but usually a dog will continue to drink and you indicate that he defecated this morning. Being a puppy he may have ingested something that he shouldn't have as well and thus is having some issues because of the ingestion.
If he consumed a non food item, he might experience discomfort leading to the symptoms you are seeing and may intiially be able to defecate what ever was remaining in his bowels but then wouldn't be. In a pup this age, I'd go ahead and have him seen. Pups this age are growing fast and need their meals. You might want to take a stool sample into the vet if possible as well.
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Since there have been recalls on certain foods, please check the following site to be sure the food your animals eat is not affected. If it is affected, contact your vet as soon as possible. Have your dog seen if they have any symptoms.