Down Syndrome Constipation

My 3 1/2 year old son has down syndrome.  When he was 6 months old he was diagnosed with hirschsprung's disease and had a rectal pull through surgery.  The surgery was definitely a success, but it's difficult for him to have a bowel movement.  He has been on miralax since he was about 15 mths old.  I MUST give him about 10 grams per day for him to be able to have a bowel movement.  I am concerned because he strains really hard every time he has a BM, even with the miralax.  His stool is very loose.  Even with "soft stool", he strains so much. Why is this so hard for him? What could be the problem?