Weakening Bowel Muscles

My 26 year old son has Becker MD.  In the last several years it has progressed a lot.  The newest issue he is having to deal with is weakened bowel muscles, causing him to strain for long periods in order to have a bowel movement.  The doctor has him taking a slow, natural approach right now by increasing his fiber intake.  So far that has not been any help.  Has anyone had this experience and what have you tried?