c/d food for 4 month old kitten?

Hi. Googling has kind of failed me here. I have a kitten and was told he needs to be on c/d food for life. I was also separately advised that he'll need to be on kitten food for a year, as he's a larger cat. I'm concerned about nutrition/calories at this age, since there doesn't appear to be urinary formulas for kittens. Lots of stuff saying any wet food is okay, then some stuff saying you're just burning money if you mix "regular" wet food in with c/d food. He has struvite crystals and a UTI caused by staphylococcus saprophyticus. Any insight is greatly appreciated.