2 cats, different diets

I could use some opinions. I have two cats. One is 13 years old and has been eating Nutro Max for many many years now. The other is a 5 month old kitten, and I have him on Innova Evo. I'd like to get the older cat eating the Evo since it's higher quality, but he refuses to eat it. I free feed kibble, so there is always bowls of kibble out for both of them. The kitten's food is within the older cats reach, but the older cats food is out of the kittens reach (until he figures out how to get up there). Eventually I know the kitten will be able to get to the older cats food, I can't think of anywhere else at this point that I can put it once he figures out how to get to where it is now.

I've tried mixing just a few pieces of Evo (literally like 5 pieces of it) in with Max's (older cat) Nutro, but he'll turn away and not want to touch anything in his dish. I've tried handing it to him as treats, but he just won't take it. Before I moved Max's dish the last time I found the kitten munching away on the Nutro. I'm tempted to just switch the kitten to Nutro, but I don't like the idea of downgrading his food. It's just a hassle to try to keep one of the dishes out of his reach.

In addition, wet food has always been a random treat but I am trying to incorporate it more in to their diets. Max's blood test results showed the levels for his kidney health at the high end of normal and they wanted to put him on a special diet for it with less protein which doesn't make much sense to me. Most people I've talked to have recommended more moisture in his diet, which is the main reason I want to get the wet food to be more than just a treat. I initially got some Nutro canned since that's the dry food he's on (got a kitten version of nutro for the kitten), but also bought some canned Evo that I'll try to get them both to eat when I run out of the Nutro. Max has only a mild interest in the wet food. I tried warming it and that seemed to help a little, but the kitten doesn't want to eat it at all. I like having kibble available 24/7, but I think this may deter them from eating the wet. How much wet should each of them be eating per day when there is kibble available as well? I feel like I'm just wasting food right now because he never finishes the wet food I put down. How long can I leave the wet food out before I toss the leftovers?

I have a 14 y/o cat who has been eating wet and dry food her whole life until recently. Her last urinalysis showed the beginning stages of kidney disease so her vet is recommending wet food ONLY. We also have a 9 month old kitten who is part pig and will eat anything that Coco leaves in her bowl (she eats about half of it at a time so sometimes he gets the other half before she does ).

My recommendation would be to stop free feeding the dry food and going to 1 or 2 measured feedings a day. They will be unhappy about that at first but they'll get used to it. Part of the reason they aren't taking to the wet food is probably because they know the dry food will always be there, so why bother. Coco is ridiculously picky, and so far the only thing she'll eat consistently is a mixture of Wellness and Fancy Feast (ugh), warmed up with a little water mixed in to make it soupy.

Since Max is so used to dry food, you could also put some canned food on top of the dry, or a little mixed in, and "trick" him that way. I leave canned food out for 3-4 hours usually, but I've left it out all day before (12+ hours) and no one has suffered any ill effects. As for how much, usually the 3oz cans say something like "2 cans per 7lbs of weight," but I'm not that lucky that Coco wants to eat 4 cans a day. Give them as much as they will eat, which in my experience has turned out to be 1 or 2 (depending on the day) cans a day, split over 2 or 3 meals. Good luck!