My 4-year old started holding his poop and it makes us all miserable. I need help.

So, our son has been potty trained for a while now. He has no problem using the big potty or his own one, in his room. He loves flushing and saying "bye, bye, poop/pee!", so it has nothing to do with fear of the toilet or something. And, he often poops without even giving notice, and I later find stinky surprises in this baby potty.He suddenly decided to hold his poop now, for no obvious reason, and it's a point of major tears and sadness several times a day. He pooped just fine a few days ago, and now refuses to go.It's -right there-! He's hunched over hiding somewhere, which is a tell-tale sign of his trying to clench that little butt as hard as he can. When I sit him on the potty he refuses to push, because he's afraid it's gonna hurt. Which, in all fairness, it probably will.We had this happen in the past, and he eventually couldn't hold it anymore and released a frighteningly huge load, and when I wiped his butt there was blood. So I know he has bad memories and his fears are justified.But, like I said, he pooped fine up until a few days ago and now just decided "nope, I'm gonna hold onto it!" Of course, this will inevitably end it a very painful and likely bloody bowel movement, which will just fuel his cycle of fear and holding. How do I break it?!?I gave him 2 small doses of milk of magnesia in hopes he would finally go, but so far nothing is happening. Honestly, at this point I don't care if he shits his pants up to his neck if that means we can move on from this. He's so sad, scared and miserable, and it just breaks my fucking heart.So, what do I do, now and in the future? via /r/Parenting http://ift.tt/2cWmyN9