4-year-old holding it..?

My 4 year old sold does the same thing. It is not unusual for him to go 2-3weeks without pooping. Finally when he does go, it would clog the toilet. After about 3 months of this we took him to see a pediatric GI. He said that there is nothing wrong with him, he is just holding it. What they find is that kids who are holding it are doing so because they are remembering some time ago when they had really bad constipation and it was painful. To them the thought of going to the bathroom is more painful than the pain they are feeling in their bellies. What we were told to do is give him 1 tsp of Metamucil Clear & Natural and 3tsp of Miralax everyday and try to get him to sit on the toilet for at least 15minutes a day three times a day to try and get him in the habit of sitting on the toilet and going to the bathroom. If he does not go within 3 days we were told to give him a suppository. What the miralax does is soften up the stool enough that it will basically slide out. By doing this it is reteaching my son that pooping does not have to hurt. We have been doing this for about 4weeks now and now we are at the point where we no longer have to give him a suppository as he is going to the bathroom on his own every 3-5days. Yes he is a boy and doesn't want to take the time out of playing to sit on the toilet for 15minutes but we are trying. The GI also mentioned that they should not be using a potty sit or training potty, they should be sitting on the regular potty as this gives the natural shape to promote going to the bathroom. I hope this is helpful