Pooping in the toilet and potty training

I have a 3 year old son who has no problem peeing the toilet, but he will hide in his closet when he has to poop and go in his pants. We have talked to the Dr. and he said it may be because it hurts so he said to give him a small dose of Miralax every morning. We tried that and he was still going in his pants and it just caused more of a mess. Does anyone have any ideas of what I can do? He hides because he doesn't want to get in trouble, so I tell him he doesn't need to hide and I love him. But I also think he is starting to get embarrassed. Any help would be greatly appreciated.