7 week old kitten had bright bloody stool earlier and now some blood in diarrhea. i've had him 1 week.?

I can't get into the vet until Wedsday and there isn't any emergency vets since I live in a really small town and my mom doesn't know what's wrong. What can we do? We have 2 older cats and when this kitten was abandoned outside we felt bad and took him in. He's seemed fine until tonight, he's still playful, eating, and drinking. I can't think of anything that could have happened, we keep him in a kennel when we're not watching him, but we have changed his food could that cause the blood and runny stool? Please tell me how I can keep him alive and pain free until Wedsday.