I'm glad to hear your cat (name?) is currently receiving IV fluids, and he should continue on them for 2-3 days min, one day is not enough.
As high/shocking as those numbers are I do know a cat diagnosed last summer with Acute Renal Failure whose initial numbers were much higher (BUN70 Cre1500), he is doing good today with numbers significantly lower and only needs subq fluids 1/week. This cat was found munching on a plant that unbeknownst to his owner was a lily, lilies are one of the most highly toxic flowers to cats.
I know you mentioned you have no plants, but what about cut flowers? A bouquet from a special occasion?
Has he eaten any grapes/raisins? (weird for a cat yes, but stranger things have been eaten)
What about suppliments, medications, asprin/tylenol, hormone replacement therapy drugs etc - including creams moisturizers etc? Any contact with any of those? A pill dropped, bottle knocked off the counter, lid not replaced, does he lick your arm after you've applied moisturizer?
Any flea/tick treatments used? Any ant bait or rat poison around?
Does your cat go outside? Antifreeze - even just the tiniest amount, windshield wiperfluid, road/sidewalk rock salt etc can all cause damage.
What food are you currently feeding?
Has your cat been on any medications/supplements lately specifically Metacam/Meloxicam or Baytril?
Does your cat have a history of kidney/bladder stones/crystals?
I see slight metabolic acidosis in the Anion gap, the body's pH level becomes too low & muscle wasting can occur - this may be part of the reason for the lazyness along with the high toxin levels.
The MCV is a little low but not so much that it would flag as anaemia yet, and it is true that most ARF cats aren't anaemic at least in the beginning.
When was the last time he had blood work done prior to this?
Let us know what the urinalysis says &
for the IV treatments
More info on ARF here: http://www.felinecrf.org/acute_renal_failure.htm