I'd need more details to help you and kitty (what's his name?) out.
Has he had his vaccinations? Is he using the litter box normally? Is he drinking ok?
Is he sleeping a usual place or has he found another spot ?
Have you recently changed his food (what kind of food are you feeding?)
Are there other cats or animals in the household?
When you look at his eyes are they bright and normal or are they showing the 3rd eyelid (from the inside corner of the eye), watery? Cloudy or dull?
Pick up the scruff (back) of the neck (but don't actually lift him) and hold it for a count of about 5. Then count how long it takes to go back into place after you let go. The longer it takes, the more likely he is dehydrated.
Do you have a vet for him ? Would getting him medical care be a problem?
Let me know
Many have forgotten this truth, but you must not forget it.
You remain responsible, forever, for what you have tamed.