Hiya,
I don't know if that's helpful at all, but Mia had blood in her poo when she was a tiny kitten. The vet said it was because her food was too rich (it was whiskas kitten), so I moved her to something different (james wellbeloved, hilife, burns, nature's menu, that kind of thing...) and she's been fine since. Hope he gets better!
Déborah xx