Update: My 10 (or 9) month old kitten's diarrhea seems to have stopped.
His name is Dalsoo - a big eater.
Since I posted the question about his diarrhea, I stopped the fish ingredient altogether, completely switched to home-made raw.
I currently give ground-recipe with some chunks in it. I am going add some chunk meal with meaty bones and all, after my cats get used to small chunks mixed in the ground meal. I don't think I can do complete frankenprey, since I cannot get the variety that others have... Even getting liver when I need is pretty challenging here.
Anyways, I am very grateful that Dalsoo's diarrhea is completely gone thanks to advice from this forum!
Isn't it fantastic? I went through the same thing with Athena, and the only thing that ever completely resolved her soft stool was switching to raw. I give a lot of credit to the members of this forum for helping guide me in that direction.