It could be lots of things. Ony a doctor can diagnose. but perhaps this will tell you how to gather information that the doctor will need to diagnose.
Is the blood bright red? Then it is likely bleeding close to or from the anus. It could be hemorrhoids, which is probably the most common causes of bleeding. Years of pushing hard, and years of constipated stool, among other problems, can cause this.
So if his stools are hard, he needs to work on that. Fruit [especially prune] juice, yogurt, over the counter stool softeners, so many things can help. When the stools are less hard, the bleeding will stop or lessen.
and hemmorhoids can be painful.
Cleanliness is so important. Dry toilet tissue can aggravate. So buy him some baby wipes, and he needs to use them after the toilet paper at every bowel movement. A good handcream, or some butt cream[as we professionals call soothing preparations for the butt] will help a lot. Rivers uses Eucerin cream, it comes in a tub, I purchased it for dry skin, but found it works great as butt cream. . Good soap and water cleanliness helps too.
See if all this helps, and you can put off seeing the doctor until the next scheduled visit. Hemmorhoid surgery is painful, and only done in extreme cases. Doctor will advise him how to stay out of the surgical suite.
A person can diagnose hemorrhoids themselves, get a mirror and look back there, if there are small little lumpy bundles around the anus, it is probably hemorrhoids.
Is the blood like coffee grounds, or looks old, or smells old[like menstrual blood}? Then you have a big problem, the bleeding is from higher up, and various bowel and stomach problems, including ulcers, might be to blame. See a doctor soon for this one.