My cat is about 10 months old. Over the past week he has been really off his food. He will only eat the freshest, tasiest food. I've had to feed him cat milk just to get him to drink something as well. Even then he doesn't eat much. He is usually quite vocal, but he has been unusually silent, and mopes about the house, mostly sleeping. We noticed a few days ago he had a swollen eyelid, possibly from fighting with other cats and yesterday he came back in with his other eyelid swollen. I've cleaned the cuts, and they are healing fine. I just don't want to waste the vets time if it's him just throwing a strop about losing a fight, but if it's serious I would never forgive myself if I did nothing.
You could phone and ask to speak to a vet nurse for advice, but I would take him in if it were me. I know cats tend not to eat much during hot weather, but the behaviour hints at something more than just sulking. Have you been able to look inside his mouth at all? If he got scratched during a scrap he may have a sore mouth making him not keen to eat iyswim. Or there may be an undiscovered wound somewhere else that might be infected.
Cats are really good at hiding pain, and wounds under their fur too as I found out with my own male cat. He had been bitten on his tail, and I would never have known until his tail became hot and swollen around the bite, when I investigated I found two puncture wounds side by side, exactly the same distance apart as cat upper canines. They were really well hidden under the fur, and had not bled. Fortunately I was able to prevent an abcess by twice daily bathing of the wounds with salt water for a few days, until it healed.
My DH had a root through his hair and found a wee scratch I hadn't seen. He wiped it with a tissue and LOADS of pus came out. I'll be taking him to the vet today. Again. I swear he is their best customer, the amount of scrapes he gets into.