Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear that your kitty has diarrhea.
Diarrhea can be due to stress, a change in diet, eating things he should not (especially if he goes out unsupervised or is fed lots of table scraps), parasites, inflammatory bowel disease or even a food
allergy. But it can also be related to metabolic organ disease (such kidney
or liver disease) due to organ wastes irritating the gut (unlikely if he is a young cat, but possible). Rarely a mass in the colon can be the cause.
If he still feels well otherwise though we can try some things at home to help with his diarrhea.
I would not feed him any food for 24 hours. This should help stop gut spasms and restore normal gut motility.
Small amounts of water given frequently are fine as he needs fluids after all that he has lost with diarrhea. You can give him pedialyte to replace electrolytes too.
Today even with the fast you can start Kao-pectate (kaolin and pectin formulation) at 1ml per pound or 1 tablespoon per 15 pounds of body weight every 6 to 8 hours. This is quite safe and will coat his irritated gastrointestinal tract as well as absorbing bacterial toxins. You can use it for several days until his stools look normal. You can find kao-pectate at the drug store.
If he has a tense painful abdomen, continues to have diarrhea with no improvement after being on kao pectate for 24 to 48 hours, becomes very lethargic, begins vomiting or runs a fever greater than 103.5F then he really must be seen by a veterinarian as soon as possible. Make sure to take a fresh stool sample with you when you go.
After his food fast start a bland diet of 2/3 boiled white skinless chicken, all fats and juices drained off the meat, mixed with 1/3 boiled, plain white rice. Feed several small meals a day. You might wish to add 1 to 2 tablespoon(s) of canned pumpkin to each meal if you are seeing mucous or blood in his stool as fiber helps soothe an irritated colon.
You can also add probiotics to his bland diet to replace the good bacteria lost with diarrhea. Good brand name products include Fortiflora or Benebac. They can be found online or at pet stores.
Once he feels better (no diarrhea for 48 hours) start mixing in his regular cat food very slowly, less bland more regular with each day. It should take a week or so to convert him back.
Ideally you should have his veterinarian check at least a couple fresh stool samples too for parasites as parasite eggs/cysts are shed intermittently and could be part of his trouble. They may be picked up on the second sample or third test if the first is negative.
Please let me know if you have any further questions.