I am sorry to hear that Fudge has been recently diagnosed as positive for Feline Immunodeficiency virus and is currently sleeping more than usual and has a dry nose.
Feline Immunodeficiency virus suppresses the immune system allowing simple infections like an upper respiratory infection (like a cold in you and I) to make a kitty much sicker than he normally would be, and can also cause organ disease, particularly kidney failure.
When very young or older cats with already weaker immune systems compared to an adult cat gets this virus it seems to hit them harder and is more likely to affect their general health.
Fudge's symptoms are pretty nonspecific and seem to point toward a kitty that either has a fever and is dehydrated or is suffering from the effects of poor organ function and dehydration. Both would lead to a poor appetite and lethargy and likely secondary dehydration.
I highly recommend that Fudge see a veterinarian and have some further testing done and have supportive care including fluids and possibly antibiotic therapy started.
In the meantime we can baby him a bit and see if we cannot help him feel better.
You can try to help him feel better by adding warm water to his food to make it smell more (they don't eat if they cannot smell) as well as making it easier to swallow in case he has a sore throat.
Also the more fluids he gets the better. Offer tuna juice, low salt chicken broth, run the tap if he likes to drink out of the sink.
You can also add tuna juice or warm broth to his meals to perk his appetite and get additional fluids into him.
If he is refusing to eat offer a bland diet for the next couple of days. A homemade bland diet consists of 1/3 boiled minced white skinless chicken and 2/3 boiled white rice. Feed this for several days and if all seems well then slowly start mixing in his regular diet, a little more at each meal. It should take about a week to convert him back once you start.
In case his symptoms are related to a respiratory take him into the bathroom with you if you run a hot bath or shower as the steam will soothe his sore throat and airways and thin any mucous in his airways as well as helping keep his mucous membranes (gums, nose and eyelids) moist and comfortable.
You can give him an amino acid supplement called L-lysine at a dose of 500mg orally twice daily. If this infection is due to Herpes this amino acid interferes with virus replication and will shorten the infection's duration and severity. Good supplements to try are made by the Viralys brand which comes in a powder to add to the food or a tasty gel.
Some lethargy is understandable, let him rest as he needs rest to get better.
If he normally goes outdoors keep him inside, especially because he can pass this to other cats should he get into a fight and he is much more susceptible to picking up infections.
If he is not feeling much better in he next 48-72 hours he must see a veterinarian.
Please let me know if you have any further questions.