Blood in my kitten's poop!

[TLJ]

Hello,
I have a four-month-old male kitten and a five-year-old female cat. I feed both of them Purina dry food; him, kitten chow, her, adult cat chow.

He's been eating her food of late, and she's been eating his. They're in two totally opposite ends of the house, so they're clearly more interested in the other's food and aren't simply confused about what belongs to who. However, over the past two days, the kitten has been making some real stinky farts and poops, and this morning we found blood in his poop. I'd describe it as a medium red--not bright, but not really dark. His poops are pretty soft and light coloured. His a-hole looks OK, though--not inflamed.

He's still playful, but he's been looking a bit sad. He's currently on Interceptor as a preventative thing for worms--he's taken two of them so far, and will take another one next month. We also are in the process of getting rid of his ear mites by using Advantage (he came from a very low-income home with like seven other cats--we didn't know he had them til we went to get him!)

This is the first time since we got him two months ago that he's had blood in his poop, though the food has not changed. As far as we were concerned, he was totally fine save for the ear mites. He's been to the vet twice for his shots and a check-up.

Should I feed him something like tuna or yogurt or something more natural? It was hard getting him to eat the Purina when we first got him (he had been eating adult cat Meow Mix since he was born!), so I'm not totally excited at the prospect of changing his food once again. Is there something I can do now, to help him out?