My 12 Year Old Son Had Complained About His Stomach

My 12 year old son had complained about his stomach hurting the past 2 days and now he is complaining that his lower back hurts on the left side. It is difficult to determine the pain level. He is autistic and has a high tolerance for pain and does not normally complain. He did not like me pressing on his back. What could it be? No other noticeable symptoms.