Oh, and another thing you may have luck with is the cat yogurt treat tubes. I’m a pet sitter and if I get a cat that won’t eat due to stress, they will usually go for one of those! Just cut the tip off the tube and slowly squeeze the yogurt out of the top and most of the time they will lick it...
Is he drinking water at all! High Creatinine levels indicate dehydration so that could be his issue. It also causes nausea and weakness. I’m speaking from experience because I personally deal with that myself. He could just be weak from dehydration and not have an appetite? I sure hope he feels...