Hi, Brandi. I used to have a roommate a very long time ago who had a child with the same problem. She was just as confused and frustrated as you sound today.
I think you need to find out why your son has regressed, gone back in time to baby-like behaviors. Has there been a new baby in the home during the past 3 years? That is the #1 reason why children do this sort of thing. Especially the first-born child feels displaced because he used to be the only child in the house, but now there is this pooping creature, and the eldest child wants the attention that the baby is getting.
If this is the case, then usually the remedy is making sure that the eldest child gets enough positive attention so that he does not try filling up the empty space in his life with negative attention.
Even if there are no new babies in the house, he may be doing this because the negative attention is filling up what he feels is an empty space.
It certainly can't hurt to get him evaluated by the doctor. If he is holding back on his bowel movements and then having an explosive bowel movement when his body can't hold it in any longer, then that needs to be treated because it's so very bad for his body and his psyche.
My old roommate's son also seemed to indicate that he didn't want to stop what he was doing and go to the bathroom, but like your son, he would never pee in his pants, so that's not the whole answer.
You may want to put him on a bathroom schedule and make him sit on the toilet at regular intervals. He will hate this, but if you make it clear, CALMLY AND LOVINGLY, that this is a consequence of his pooping in his pants, then that will help him choose to stop doing the behavior. Bribing him to stop the pants -pooping hasn't worked, so you need to give him an unpleasant consequence, a discipline, for pooping on himself.
Another strategy is to make him wash out his own pants everytime he poops himself. See, right now, he's getting negative attention from everyone for doing it, and no real consequences because the attention is what he wants. On top of that, he gets YOU to clean up after him! Maybe he's giving you a little bit of punishment there, another gift to him. Take away his joy, his reward, for pooping himself, and he should stop doing it. Washing out poop from his own underwear and slacks is very unpleasant, so this is just a natural consequence and not a harsh or unusual punishment.
On top of that -- timing him and making him wash out his own poop -- you will have to go back to praising and rewarding him for starting to poop in the toilet again. Not bribing -- wait til he makes it to the toilet and reward him for doing so. And stop letting him know how hateful this is to you. That adds to the negative attention by showing him how stressful this is for you.
He may be mad at you for something. Try finding out what that is, and make amends before this becomes a deeply set lifelong problem between you.
I certainly hope that the whole family comes together and this problem gets healed.
Peace,
Syl