is the blood red?? its probably nothing. how old is he?? if the blood is bright red then it is more then likely hemmorhoids, or a fissure. is the blood mixed through the stool, or on the toilet paper afterwards. if its mixed in the stool, that is more cause for concern because that means he's bleeding up further then a hemmorhoids would show.
It could be an internal hemorrhoid, colitis, or something else. It's not necessarily something terribly serious, but it should be checked out if it doesn't subside. There are lots of threads on this forum dealing with this subject, so you can see it's a fairly common problem.