How old is he?
Does he go outdoors?
Is he able to urinate fine?
What and when did he eat last?
I am quite concerned about your boy. Such a drastic change is not normal.
He may have been injured, ingested a toxin or hurt by an evil human.He could have goen something on his fur and licked it off.He may have drank antifreeze.
I am glad he is eating. Keep him indoors today where you can observe him.Make sure he is able to urinate.
Offer him canned cat food, tuna, baby food (no onion powder) etc.Try to get him to drink tuna water or milk.
If he wont eat, has pale gum, cannot urinate or gets any worse he need to see a vet.
Please let me know how he is doing. I hope he feels better very soon.
Sincerest best wishes,