I am sorry to hear that your fellow Sir isn't feeling well, refusing to eat normally and very lethargic despite normal blood tests and lack of a fever. It is frustrating to hear that test results are normal when he obviously is not.
I am worried that he has suddenly lost his appetite. Cats that don't eat well for a period of time can get into trouble with hepatic lipidosis. This is fatty liver disease caused by overwhelming the liver with fats being broken down for energy to live when they don't eat such that it cannot perform normal functions. This is a secondary disease, but it can be deadly and needs to be aggressively managed. If a cat doesn't eat anything for more than 3-4 days it is time to consider placing a feeding tube.
I understand that blood tests have been done. I assume that he had a complete blood count and biochemistry profile as well as a T-4.
If it hasn't been done I would also check a specific blood test for pancreatitis, a fel spec PL (feline specific pancreatic lipase).
If those tests are done and come back normal the next step is an abdominal ultrasound and/or either exploratory surgery or endoscopy to gather biopsies of his gastrointestinal tract to look for inflammatory bowel disease or cancer.
In the meantime since he is showing signs of nausea and stomach upset (lack of appetite) you can give him an acid reducer to help. At home you can give either:
1) Pepcid ac (famotidine) at a dose of 1/2 of a 10mg tablet per 10-20 pound cat every 12 hours.
2) Prilosec (omeprazole) at a dose of 1/4 of a 20mg tablet per 10-20 pound cat every 24 hours.
These are acid reducers and could help him feel better. They are quite safe and should be given for several days or long term if need be.
You can tempt him with a homemade bland diet for the next several days. A homemade bland diet is a mix of 2/3 boiled minced white skinless chicken and 1/3 boiled white rice mixed with some low salt chicken broth to make it easy to lap up and swallow and get in plenty of fluids. Feed small meals frequently.
Another option is to try feeding him a higher calorie prescription food called Hills a/d or Iams Maximum Calorie from your veterinarian.
Make sure he has access to plenty of fresh, clean water. Hydration is extremely important, especially in sick cats.
Please let me know if you have any further questions.