Cat with diarrhea

One of my cats, Samson, has been having diarrhea for about 4 days. It smells absolutely terrible and I am concerned about him. One of the problems is that we don't have money to take him to the vet again. He was there about 2wks ago and had bronchitis and a minor yeast infection in his ear. They gave him an antibiotic and fluids and he's back to himself, aside from the diarrhea.
We have forti flora from the vet because my other cat, Macy, had diarrhea when we first adopted her (which was about 3wks ago). They did a stool sample for her and it came back negative for anything. However, Samson will not eat the forti flora. He will actually stop eating if I put it on his food. We feed him dry cat food and have not switched brands or anything.
Once again, I am very worried and was wondering what could be causing it. Samson and Macy seem to really like each other, sometimes he'll even lay next to her and clean her so I don't know if it's stress related.
I read that I should not allow him food for the next 12-24hrs so I think I am going to try that. I have also read that unflavored pedialyte could be good to give him incase he's dehydrated. Does anyone have any other good home remedies?
Thanks!

I would definitely not withhold food. Cats can become sick very quickly without eating.

You can give  him boiled chicken and white rice for a  few days and see if that helps him.  Don't use any spices or salt or anything when boiling the chicken, then shred it up and mix it with the rice.  You can use the water you boiled the chicken in to add moisture to this mixture. 

This is not nutritionally complete, but is ok to feed a cat with diarrhea for several day, if necessary.  Also, you might try giving him a pro-biotic other than Fortiflora.  Try a human one that you can find in a health food store (maybe you are already taking one?)  I take the same ones I feed my cats.  They are tasteless, so you can mix 1/2 - 1 capsule into his food once a day.  Or give him some yogurt with live cultures.  Flavored is ok, but NOT diet.  I usually give mine plain, but some cats like flavored.  (yogurt contains some of the same things as probiotics, but not in the same quantities)