Thank you for the query.
Blood in stool
can be caused by anal tear (fissure) or hemorrhoids, but such blood is usually on the surface of the stool and appears on the toilet paper as well.
Blood mixed with stool is a symptom which for sure needs further investigation. The blood is mixed because the bleeding source is far enough in the large intestine so the stool and the blood has time to get mixed. It can be caused by inflammatory bowel disease
, Crohns disease
), bacterial infection (usually diarrhea appears), large intestine polyps (in such young age it is only possible if hereditary disease is present like FAP or Juvenile polyposis).
I suggest you to visit gastroenterologist and have colonoscopy
for your son performed.
Hope this will help.