oh oh oh...a bum question I know the answer to...wahooo....
it's called an anal fissure (A a small tear or cut in the skin lining the anus which can cause pain and/or bleeding.). It's usually caused by hard poo, though somewhat ironically runny poo can cause it too (don't ask me why). The main things to do are make sure he has lots of fluid and isn't blocked up. Regular baths with a little bit of tea tree oil are great to soak in - just make sure that none of the oiled up water gets into his eyes. Sudocrem is also great for healing them up. They can take up to a week to heal. (Anal fissures in infants usually self-heal without anything more than frequently changing diapers and treating constipation if the cause.)
Okay, so I'm guessing that my intrigue levels have increased due to being able to answer this one...