Sounds like it's time for a trip to the vet & to get a clearer picture of what the "kidney problem" is & a treatment course for it. A full panel blood & urine test would be necessary for an accurate diagnosis.
What food is Simba eating? If he's on dry food, now would be the time to switch him to a canned only or raw food diet, especially given the kidney problem. All cats with any type of kidney issue need plently of fluids in their diet to give the kidneys help to flush out toxins.
& let us know what the vet says