Have posted here a few times with ongoing trials of potty training DS1.
He finally got weeing in potty and toilet sometime after his third birthday [after about a year of potty training!].
But he will not/cannot poo in potty or toilet.
He just "wants poo to go away". He will hold onto it for ages, sometimes for a couple of days.
I don't understand how he is managing this.
He will be walking hobble legged with his bum in the air and a strained look on his face. Clutching his bum and actually trying to push it back up [tmi!]
It looks like a poo is imminent. I can encourage him onto the potty or toilet and then it is like the urge completely leaves him. He suddenly looks calm and relaxed. Will sit there for a while and then gets off not having produced a thing. Yesterday I even tried the bubble thing, getting him to blow bubbles because apparently they can't hold the poo in whilst doing this. But still nothing!
At the moment he has been holding a poo for about 2 days. Yesterday it looked like he was about to do one a number of times but somehow held it in.
And again this morning.... but now he is sitting watching tv and looks completely comfortable 
He sometimes says he wants a nappy [pull up] to poo in and sometimes I can encourage the potty. But still nothing. I am worried the poo is getting harder and more and more difficult to push out. He does say on occasion that poo hurts and "is not his friend"
and 
Does anyone have any advice or reassurance? Please!