What can i do for a cat with running poo?

He was a stray we took in until the shelter has room. We are taking him in next week. But until then. I assume its worms. Will pumpkin help? Is it ok to give him probiotics? I feel so bad for him. I am feeding him wet and dry food. I am wondering how much could be the food. There was blood the first time I saw his poop. But havent seen any since. I have also found white specs on his fur, I just feel so bad that he has to wait til next week to get help. Anything I can do for him? I am adding water to his wet food so hopefully he doesnt get dehydrated. He does drink from the water fountain. 

Royal Canin Spay/Neuter has the highest fibre content going for commercially available cat foods, and most cats will eat this. I have used this with foster kittens with severe diarrhea and have had excellent results, often within a few hours. I'm talking dry food, not canned. If the Spay/Neuter works, after a few days, ease him onto a plain wet food if you don't want to feed dry permanently. This food saved my little Montgomery's life, and several rescues in the area use it for this purpose.

I dont have money or a place to buy this over a weekend. I am just trying to make him comfortable til tuesday,