Potty Training

Grayden is 3 years old. He will be 4 in June. He has been "potty trained" for quite awhile now. Except poop:( He will wake up in the middle of the night to use the restroom and everything. He just wont poop in the toilet. He wants to put on a diaper to poop. I dont understand it. Ive tried every form of bribery and reward imaginable. Ive been the understanding mommy, Ive been the mean mommy. I dont know what to do. Im at my wits end. The only time he has ever pooped on the potty is when I gave him and enema. I have done that twice now. Anyone have any suggestions?