OTJ cat with kidney issues?

Mali is 17-1/2 years old, has been diet-controlled and off insulin for almost 6 months now. He had a full blood and urine panel at the vet yesterday. I got a message on my answering machine from the vet today saying his numbers were up a little for his kidneys and we may need to discuss a "kidney diet" change. Everything else was fine except she said the test showed slight dehydration (although she said during the exam he didn't seem dehydrated). My vet won't be back in until Friday so I did a little research and have some questions that maybe some of you can help with.

Mali gets stressed easily and peed in his carrier on the way to the vet and again when they were getting ready to take blood, could the fact that he peed twice right before the tests have affected the results? Is that why it showed he was slightly dehydrated?

He eats CLASSIC Fancy Feast, mostly fish flavors (he's a picky eater), and I usually add a couple teaspoons of water to his food. I've started adding more water to his food to make it more like thick soup and he's eating it well. I know fish flavors aren't the best thing to feed, but are any better than others (like is salmon better than tuna, etc...)?

I know sodium and phosphorus intake should be kept low, any suggestions on Fancy Feast or Friskies flavors that are low in carbs, phosphorus and sodium? Mali won't eat the fancier foods like Wellness, Hill's, etc...

What about supplements? I read that "n-3 fatty acids" are good for kidneys, can he get that from fish oil capsules? Is fish oil high in phosphorus? Any particular types of fish oil that are best?

I've been thinking about trying Fortiflora just because he's older and has very long fur that knots easily, I figured it would be good to give him some extra vitamins. Is there any reason why I shouldn't give a cat with the beginnings of kidney disease Fortiflora?

Any advice would be appreciated, thanks!

 

Just bumping this one, since I need some of the same answers. Is fish oil good for cats with kidney issues?