My son ; aged 5 yrs old, complained of pain on his left side of his waist ( stomach ). what is the problem & possible causes?

Usually constipation: Right sided pain usually worry about gall bladder or appendicitis. On the left side the organs are colon, spleen, part of pancreas, part of stomach, and left kidney. Without associated fever or trauma, the most common cause is bowel gas and constipation. If persists, then should consult pediatrician for physical examination and possibly imaging.