I have a Himalayan who was just like that as a kitten. My vet suggested we put him on a food that was grain free, some cats just have issues with that element in their diet.
I tried a few different ones, Natural Balance Limited Ingredient Diet (that one was about $13 a bag but lasted a month) and then another one called "Taste of the Wild" that my cat still loves, it's completely grain free, he likes the Roasted Venison and Salmon. It's $12 a bag and lasts for a month.
He loves the food but because it has so much protein and no empty calories from grain and corn, he doesn't have to eat as much.
It completely transformed him, he doesn't have the poop on his fur anymore and he's really muscular and his coat is so shiny so it's a great brand of food for him.
Try around and see what foods work for your cat, it's sometimes just trial and error but higher-end ingredients do seem to help with cats that have digestive issues. I hope that helps your really adorable kitty!