Take him to the vet.
Is he hurt in anyway. If yes them give him a day or so to feel better, if his condition gets worse take him in to the vet. If their are no clear injuries on the outside of his body then try to give him water and keep him safe inside away from other pets. Give him/her space if they need it. Wait 24 hours and try different foods. Maybe he is just picky. Either way if within 24 to 48 hours the dog hasn't eaten then take him/her in.
The staff is will qualify ,Dr. Williamson was great
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