Not sure about Simba's details, but kitten food is very high in protein, which may not be good for an adult cat because it puts more stress on kidneys. If he's eating well, just give him more good cat food, like Whiskas (which is what my old ladies have eaten all their lives, well, while they've been with me, no advertising intended) or perhaps give lots of cat milk or go to the butcher's and get some cooked meat. When Polly was last ill, I got her through it by giving her sliced turkey....and I'm a veggie! It meant going into that dreadful shop and seeing those fresh cream cakes......
That's a thought, will Simba drink cream. Try a few extra cat treats.
I don't know his medical history though.
