I'm so sorry to hear that Sugar is so ill.
I've seen cats live on hydration treatments with diet modifications for months. What can be done to get fluid into cats is an IV, the vet leaves a port in the cat's leg you plug in daily or every other day to give sterile fluid in prescribed amounts. I've also seen a vet give a large bolus of fluid under the skin, a clysis, every other day to keep an animal hydrated.
Diet is very important, a renal diet that the vet figures out will help decrease those 2 high levels. Probably the BUN and the creatinine, 2 measures of kidney function.
The supplements I'm afarid I don't know enough about to advise you, but I can ask the vets I work with if you like.
Thank you for your advice and thank you everyone for your well wishes! Sugar is such a loving kitty and I hate to see him suffering. I posted his lab results (blood work results) in my thread asking for advice about my Sugar. Here is one of my favorite photos of him, which I took last November:
