Are you sure that it was blood? Anything that he may eat or drink that is red can cause a discoloration. If that's not the case...was the stool maybe "too large" for him to pass? He could possibly have a tear or just simply strained too hard. I would just keep a close eye on it, and if you continue to see it, most definately take him to his Pedi.
What has he been eating some food can cause this, beets are one, watch for it to happen again, is he constipated ,straining when he goes.If you are concerned and its not food ,happens again, it is always good to get it checked out.