I applaud you for your caring actions and intent. So this is what I suggest.
Try not to ask about the pain. I would show empathy when the pain comes but would ask about feelings instead. There is nothing wrong with the body. The symptoms are being created by unmanaged stress, mostly repressed, and personality type. When symptoms occur, gently guide him back to the emotions and away from the pain and physical preoccupation.
Remind him that there is nothing physically wrong and help him to not feel disappointment when the pain comes as it will for the time being. With acceptance and time the pain will go.