It sounds like you may be dealing with kidney
failure or it could be something else (liver disease, cancer, pancreatitis, IBD- inflammatory bowel disease).
If he's that lethargic, then sub Q fluids may not be enough. He may need a day or two in the hospital on IV fluids to rehydrate him. He may also need meds to help stimulate his appetite and treat the disease process more specifically.
With a normal BUN, I would guess it's something else causing the signs. Usually cats with kidney failure will have increased kidney enzymes (BUN and Creatinine). They will also have dilute urine and often an elevated Phosphorous.
If this is a type of cancer, then she may need steroids (Prednisone) to start treating.
If pancreatitis, then IV fluids, antibiotics and pain meds are needed. Those cats typically take 2-3 days in the hospital to get over an acute attack.
I hope this helps, let me know if you have any other questions.