Does he seem to feel bad? You can take his temperature rectally, with a human thermometer- normal for a resting dog is 101-102. Any vomiting, diarrhea or lethargy? Dogs can go several days without eating with no problems (unlike cats) so I would not be too concerned just yet unless he is showing other signs. If he continues to not eat or he shows any of the above symptoms then a vet check would be a good idea. Is he drinking? Maybe mix a little broth in with it and see if that entices him. If so, then he is probably just being picky and you can hold out a little longer.
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