A. Diverticulosis is largely a condition that affects older people and I think that it is not very common in children or teens. Adults with diverticulosis can have mild cramps, bloating, and constipation. They can also develop diverticulitis, with tenderness around the left side of their lower abdomen, fever, nausea, vomiting, chills, cramping, and constipation. Complications of diverticulitis can include bleeding, infections, perforations or tears, or blockages. However, diverticulosis doesn't sound like a probable cause of his symptoms.
The fact that he has bouts of diarrhea and constipation might point to irritable bowel syndrome (IBS) as a cause of his problems. Symptoms of IBS can include abdominal pain, constipation, diarrhea, or alternating constipation and diarrhea. Bleeding and weight loss are not symptoms of IBS though.
Celiac disease is another condition that could cause symptoms like your child is having, in response to having an intolerance to gluten, which is found in wheat, rye, and barley. Symptoms of celiac disease can include recurring abdominal bloating and pain, chronic diarrhea (which can be bloody), weight loss, and fatigue. Measuring blood levels of antibodies to endomysium and tissue transglutaminase can help doctors to diagnose celiac disease, although a biopsy of the small intestine is even better.
Inflammatory bowel disease (IBD), which can cause inflammation in the small intestine and colon, fits his pattern of symptoms very closely. The two types of IBD include Crohn's disease and ulcerative colitis. Both can occur in older teens and young adults and they cause similar symptoms, with abdominal pain, often in the lower right area, diarrhea, rectal bleeding, weight loss, fatigue, and sometimes fever.
In addition to blood tests, stool studies to look for infections, and an Upper GI, children with the symptoms you describe may require a barium enema xray of the colon and/or colonoscopy, with an intestinal biopsy.
A Pediatric Gastroenterologist should be able to help further evaluate and manage your child and figure out what is causing his symptoms.
You will likely also find a lot more information about these conditions from the About Guide to Inflammatory Bowel Disease.