If you are not seeing any blood, you can try
to prepare a bland diet for your cat
using white rice and chicken. Try
to not give him any food for about 12 hours, only water, and then give him some small amounts of cooked white rice and boiled chicken. It is important that he only take a small amount (a couple of spoon fulls at most). If the diarrhea appears to be resolving, feed him the bland diet every 3-4 hours, increasing the amount fed slowly.