I was thinking too myabe he is coming down with something I find going off food often a first sign.
Where you live? Cause some states suddenly got warm so that may've put him off food, I get funny tummy when it gets hot.
Umm.. what does he say when you ask him about it? Does he say anything?
Maybe try let him help you cook something then he may want to eat it cause you made it together.
Would he like cordial? You could buy ice tray with the shapes so he can have "special ice" in his drink or just to eat.

