My 17 year old son has blood in his stool and goes to the bathroom after each meal. He says he feels good. What could it be.?

Frequent? It is not normal to bleed from the anus. This will need to be evaluated by an endoscopist to rule out inflammatory bowel disease. However, it just may turn out to be hemorrhoids.
Probably benign C Dr. Feeling good (if true) + absence of abdominal pain or pain on defecation means unlikely inflammatory bowel disease or anal fissure. Could be internal hemorrhoids, benign polyp hemangioma, carcinoid etc.. Age in his favor for benign condition, however needs work-up including proctoscopy/colonoscopy to establish diagnosis and rule out rarely occurring malignancy.
See your doctor. There are many cause of blood in the stools. Hemorrhoids and anal fissures can cause bloody stools without symptoms. However, croh's disease and ulcerative colitis could cause this as well. Both of these aren't commonly diagnosed as early as 17 but it is not out of the question. You should see your doctor for a true diagnosis.