6 year old WON'T poop in toilet

  I have a 6 year old boy who was doing quite well using the toilet and when I told him it was time for him to start wiping himself everything turned on a dime.  He will not use the toilet anymore and could care less if he sat in it all day long.  I'm at the end of my rope.  I feel I have done everything I can from candy rewards and stickers on calendars to taking him to the pediatrician and having him X-rayed.  His doctors report said he was impacted, so he was put on a prescription laxative for a month, he said this should make thing right.  Well, it helped clean him out but we are right back where we start at.  I have know idea what to do, I have tried to just let it be and make him clean himself up and he did.  I decided to put him in pull-ups because I was tired of cleaning up his underpants everyday  and thought maybe having diapers on again would be to embarrassing for him but I was really wrong.  He would just go and get a plastic bag after he was done soiling himself take off his pull-up put it in the bag and take it out to the trash.  So, now what?  I am embarrassed for him, I don't know what to do.

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