What an awful start this poor baby had. I am so glad he found such a womderful home with you!
He may have coccidia or giardia.These protozoan parasites run rampant in shelters and outdoors and are very common in kittens.Most are born with them.
Metronidazole (flagyl) is scripted for these parasites. It must be given for ten straight days - the off a week and repeat another ten days.That is because the parasites shed cysts (eggs) that do not mature in a week hence the cat keeps becoming reinfected.
Because of this lack of shedding into every stool, three separate tests are required to rule it out.
Of course stress is a factor. Surgery weakens the immune system and allows opportunistic bacteria to take over.
He may just have bad bacteria.
Give him one half teaspoon of plain yogurt every day. The acidophilus will keep "good" bacteria in his colon.
If he stops eating, begin by giving him two droppers (cc size) of unflavored pedialyte every ten minutes for an hour. Next water down(plain water) canned cat food or baby food chicken or turkey with no onion or garlic and give only one dropper every ten minutes for another hour. Wait two hours and repeat both. When he feels hydrated he will lick up the gruel on his own.Add infant rice cereal to his food - just a bit.That will firm the stool.
Make a temp electrolytes solution by adding an eighth of a tsp each of salt, sugar and pancake syrup to 12 oz of water.You can use tuna water to hydrate as well
Cats cannot take any human OTC's because the sensitive to the ingredients so we have to treat him holistically.
It is very important to keep him hydrated with the pedialyte.
Apply a dab of vaseline to his anus daily - it will keep the area lubricated.
Please let me know how your new baby is doing. I will always be here for both of you.
May God bless you for saving him!
Warmest best wishes,