Hi there,
How is your kitty acting sick? vomiting or diarrhea?
How long has he been ill?
How old is he? Is he neutered?
Does he go outside?
Why did you switch his food? How long ago did you switch it? What brand was he eating before and what brand is he eating now?
How is he acting now (lethargic, not wanting to move, or running and playing?)
Let me know and I will try to help.....