Bowel movement

My four year old son still struggling with bowel movement. He mastered peeing in the toilet right away but he holds his bowel until it hurts and cramps his stomach. We tried every trick in the book, charting, bribing, encouragement,...etc. We also used  laxatives, Enema,and  glycerine. He is still able to hold it in even if he is cramping and very soar. He tells us that he is scared to poop in the toilet and he doesn't poop in his pants because he gets embarrassed so he holds it in.
He can hold it for up to 8 days while we give him laxatives everyday to give him the urge to push soft stool so he won't get cut. One day he could hold laxatives and glycerine was given to him at the same day and he still can hold it.
We are very worried about him, we don't know what else we can do to encourage him to do it in the toilet. He is a boy that if he gets something in his mind he wants to accomplish he does it with great ease, but this issue he is not totally convinced and when he does it he becomes very proud of himself. We need advice with this issue, its really concerning us.