Is he drinking normally? I'd be more concerned if he was refusing water too. If he's not drinking, I'd call your vet asap.

How's the weather where you live? Some dogs won't eat when it's really warm out.

Is he free choice fed or on a schedule? Is he eating treats?

If his behavior is otherwise normal, I wouldn't panic yet. If he continues to refuse to eat for a couple more days or his behavior changes, I'd get him to the vet.