Lower left abdominal discomfort in men. Will an u/s scan help rule out cancer or diagnose it?

Diverticulitis +++. Possibilities are inflammation in little outpouches in sigmoid colon (diverticulae); irritable bowel syndrome (spastic bowel); ultrasound limited study. May need ct scan or colonoscopy if persists.
Evaluation required. Your symptoms need further evaluation by a health care professional. Obtaining a history from you and a physical examination are the first steps in figuring out what is wrong. An ultrasound may or may not be necessary. You should not jump to the conclusion that you have cancer. Diverticulosis or diverticulitis are more common. Please see your health care provider.
Possibly. There are many causes of lower abdominal pain in men. Your age, other medical conditions, and symptom complex will all help to guide imaging. Quality, quantity, type and duration of the pain, relationship to activity, bowel movements, urination, etc, will all aid in trying to suggest a work-up. Referred pain also needs to be considered. Ultrasound may be included in the work-up.