I agree with adding water to the dogfood to make it softer and aromatic. It might be just a sore mouth. However, make it available for only ten minutes, then throw it out. When he figures out it won’t be there later, he’ll get the point.
Other dogs in the house also get some dogs to eat. they have no interest in food until they think the other dog may want it. I think it’s mostly a pain issue, that will be better in a week.
I thought this title was “How do I get a new dog to eat?”