There is two alternatves here,your cat have a bad tooth and the infection(and pain)makes eating problematic.

Or your cat have got a scratch that have become infected giving your cat pain when he tries to eat,The infection itself might make your cat ill so he is loosing his appetite.

It looks like your cat have got an abcess this is very common in cats and your vet will need to open it up and flush it clean with salt water,if this is from a scratch your cat will probably not need antibiotics.

If it comes from a bad tooth your vet will remove the tooth and give your cat antibiotics and a painkiller.

Please do not worry this is easy to fix for your vet,i am sure this is less serious than you might think.

There are two alternatives here: your cat has a bad tooth and the infection  (and pain) makes eating problematic.

Or your cat has got a scratch that has become infected giving your cat pain when he tries to eat. The infection itself might make your cat ill so he is loosing his appetite.

It looks like your cat has got an abcess. This is very common in cats and your vet will need to open it up and flush it clean with salt water. If this is from a scratch your cat probably won't need antibiotics.

If it comes from a bad tooth your vet will remove the tooth and give your cat antibiotics and a painkiller.

Please do not worry, this is easy to fix for your vet. I'm sure this is less serious than you might think.