nausea

My adult son has diagnosis of ME/CFS.  
He suffers with severe nausea after defacating.  He has no pain, bloating, diahorrea, constipation or nausea before evacuation.  His ME has caused severe muscle spasms of his back, footdrop, temporary paralysis, debilitating fatigue.    
Can you suggest what may be causing this nausea after bowel movement or suggest any helpful medication.  He has tried metroclopramide with no success.  He also suffers with travel sickness very easily.
Thanks for your help.
Con.