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I am sorry to hear you are worried about Freddie. My name is Rebecca. I am a veterinarian and will do my best to help.
Anytime a cat goes a day without eating, I am worried about internal illness or infection. Cats tend to be secretive about not feeling well, so if a cat won't eat he is saying that he does not feel good. If you check his gums, do they look a normal pink color or do they seem pale? Can you take his temperature?
If he were my own cat, I would begin with a good physical examination. In addition to checking his gums and taking his temperature, I would palpate his internal organs and lymph nodes, listen to his heart and lungs, check his reflexes and pulses.
Is he drinking at least? If he is not drinking he may be getting dehydrated which complicate whatever making him sick. You can check him for dehydration by gently pinching the skin over his shoulders and letting go. It should go right back into position in less than one second; if it takes longer he is dehydrated and probably needs to see a veterinarian to get supplemental fluids (IV or injected under her skin). Try to encourage drinking by offering water as well as broth or Pedialyte.
Let me know what else I can help with or answer.
I hope you were able to see what I wrote. Let me know how he is doing.