I am sorry to hear Rusty is not well. How long has he been like this?
My name is Rebecca. I am a veterinarian and will do my best to help.
Is he eating anything at all, and is he drinking? Can you find out if his gums are pink in color, or if they look pale?
Does he go outside?
He could have a fever, infection, parasites, or he could be injured especially if he goes outside. If I could, I would take his temperature, check his gum color, and see if he is dehydrated. If he is dehydrated, if his gums are pale, if his temperature is high, he may need medication and treatment from a veterinarian.
Try to keep him hydrated by encouraging him to drink water, milk, chicken broth or tuna juice. If he won't even take liquids, he may need medical attention.
I always worry about feline leukemia virus in young cats, and that can be serious. It can cause fever, anemia (pale gums), loss of appetite, etc.
Let me know what other questions I can answer.