Potty training 4-year-old with encopresis

Hi, I am new here but am looking forward to exchanging information with all of you wonderful mommies!  Here is my question.  I have a four-year-old boy (he will be five in July) who has been diagnosed with megacolon and encopresis.  Basically, he was terribly constipated early in life and has passed stool of massive size all his life, but his condition went undiagnosed until age three, when his colon was so far stretched out of shape that he no longer could feel the urge to poop.  After being treated with Miralax and Lactulose, he is doing better; however, he has learned that pooping hurts and so he is still very hesitant about using the potty.  We can get him to sit on the potty now, but I think he is still holding his poop in as he never has results.  Sometimes, I see him sitting on the floor in his pull-up, and I see the grunting, etc. and know that he is pooping, but he denies that he is and refuses to go sit on the potty at all during those times.

Has anyone out there been through this who can coach me on how to get him to sit on the potty and release his poop?  He is supposed to go to kindergarten in the fall but they won't take him unless he is potty trained.  The clock is ticking!  Please help.  Thanks a bunch!

My daughter has this issue.  She is on miralax.  I give her enough that she can't hold it more than one day but it's not diarrhea either.  When the miralax started making her a little regular she was still in a diaper, so now that she's potty training I know about when she needs to go.  At that time (and when I see her starting to make faces and noises) we sit on the potty FOREVER!  I put her little one in the living room and we watch cartoons, I read books, we sing, I give her snacks....whatever I can do to get her to sit there.  Most of the time she wants to get off of the toilet when she feels the stool coming, and then I end up grabbing her and putting her on AS it's coming.  If there's some in her underwear, I put that in the toilet too.  Anything to show her that poop goes in the potty.  On the occassions where she actually goes in the potty, we make it a party.  We have songs and dancing and candy and cookies, etc.  I'm hoping that eventually she'll realize how much easier it is to just go in the toilet.  Oh yeah, and prayer....we pray A LOT!!!!