My 13 Year Old Son Has Pain On His Left

My 13 year old son has pain on his left side when it is pressed. He started getting motion sickness while driving and throwing up randomly. Doctor said he is constipated. His gastro had him taking 2 tbsp of mineral oil morning and night and 1 tbsp of myralax every morning and night. He has the urge to go to the bathroom for up to an hour after his bowel movements, so he is always in the restroom. He has done this for 6 weeks. His left side still hurts when pressed on. What could this be?