
Hi,
What was he eating before, when he was fat before Christmas? Has the vet checked his teeth? If his mouth is sore that could put him off. A liver function blood test might be a good idea too.
It may be that with food available all the time it has lost its value to him. Try not to worry (easy to say, I know) but your stress will transfer to him and worry him. Put his food down, sit beside him and read a book or something. Just ignore him and hopefully he'll get on with it. Then after 15 minutes (unless he's actually eating) pick up his bowl and take it away. Do this at breakfast, lunch and tea. With luck that may be enough to give food back its excitement.
Good luck.