Your concern is understandable. If, however, it has been eating less or not been eating much for a day or two, that's nothing to worry about. Dogs usually stop eating temporarily due to stress, diarrhea, when they are in heat and due to warm weather. If it's altogether stopped eating then you must take it to the vet. There are loads of medical reasons due to which dogs stop eating, like pain in the teeth, allergy to food, death of a packmate, or in some cases dementia(they forget how to eat). You have to take it to the vet to determine which one of these is causing the loss of appetite in your dog.

 I agree.. If it's just been since he got the shot... No worries.. It probably just made him nauseous, and he'll start feeding when he's feeling better... If it's been longer than that... Consult with a vet.  As long as he's willing to drink, he can go a long time without food without any harm... Try offering him some warmed broth... And then some warmed broth that you've cooked a little rice in (still soupy)... Sometimes that works with an upset stomach, and helps to get them past it.