Cat with diarrhea

I have a nine month old cat that has had diarrhea for months.leaking all over the house, his poor butt is raw amd swollen...
I've taken him to the vet several times and he's been on several different medication.
I forget the first one, then, metronidazole, prednisone, last one was panacur...
He acts normal, playing, eating, drinking and everything...I've dewormed him and switched him for cat chow to a cat food with probiotics and without corn or wheat..none of my other cats have diarrhea and I don't know qhat else to try...I'll be calling my vet tomorrow and last time he mentioned maybe another round of panacur....any help is greatly appreciated...oh, he's also utd on shots and has been tested For fiv.

Oh, as a note his sister has a weakened immune system (she is always getting colds-yes she to has seen the vet) ...so maybe he has a weak immune system that affects his gut...again, please any tips and ideas will be greatly appreciated.

I used this with my little Louie Bear when he was a kittnen on the vet's recommendation. He's still eating it now, and loves it, and his 'problem' has largely cleared up.

http://www.candioli-vet.it/index.php/en/piccoli-animali-/feeds/item/5-florentero

Do you use the tablet or paste? I'm gonna look into buying it. Seriously, I'm willing to try anything!

So I haven't yet purchased the recommended above. But the vet uped the dose of panacur, i gave it four days, he then had me give prednisone for four or five days...I'm. Suppose to call tomorrow and update him..no change. In doing research on diarrhea and someome mentioned lysin (how ever you spell it) anyone heard of it used for diarrhea?

Raw food.