Yikes! Poor little one
Have you offered any dried cat food? Most people feed a high quality cat food (first ingredient actual meat, protein around 28-30%). My Thistle's poop was noticeably healthier when I switched her from cheap crap she ate when I got her to a high quality cat food.
Also, I've heard of Small Animal Benebac being added to food to help with tummy issues, it's a probiotic you can find at pet supply stores.
I've never tried it, but I've also heard of people giving Pedialyte to help with dehydration, which is going to be one of the biggest concerns with diarrhea.
Good luck with everything! Fingers and quills crossed for your little one to pull through!