Blood in Stool: Potential Causes & When to See a Doctor

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Answer
It could mean many things.  If he has been constipated it could be an anal fissure.  (a tear in the anus) If he has hemmroids those could be bleeding.
It could be from a tumor such as cancer!   There are several things, I can only give you possibilites he would have to see a Dr. to know for sure and I urge him to do that.
Good luck to you.