sorry for kitties ongoing problems...as with any chronic health issue the exact cause and treatment can be difficult to find.
Psyllium is a soluble fiber and therefore good for diarrhea, kitty can become dehydrated if the diarrhea is frequent and watery....than as Nancy suggested SubQ fluids via your Vet would be required.
make sure your kitty is drinking plenty of fluids, a chicken broth (without salt, preferably homemade) is soothing and will encourage drinking.
Also Slippery Elm Bark avail at any health food store, I use it for my Nemo for his urinary tract health, its also soothing for the bowel muscles.
I could give you the safe brand names for cats if you are interested?
Probiotics(acidophilus) to improve the nutrient absorption, with diarrhea the food is passing quickly through the digestive tract.(this is avail thru a health food outlet)
I hope the new diet and supplementing esp with psyillum will give him some relief.
Please keep us posted.
I forgot to mention he's 11 months old.
One of my cats (she is now 10 years old) had corona virus as a kitten. It is very persistant and can take quite a while to overcome and even then there are typically lasting effects. My can still reacts to any type of vaccines and her growth is stunted. Your kitten will probably need subquetanious hydration and force feeding for a while.