Well my son had the same issue
What i did was switch to cloth diapers
http://www.gdiapers.com/
It worked well, even thru the night.
When he pooped, we would dump the poop in the big potty and let him see where it goes, I always let him flush, then I swish the cloth diaper in the toilet water to get the rest off the poop off. and then let him flush.
seeing this really changed his mind set on where poop goes.
then during the day heavy training under pants.
and a sip of my morning coffee.
this helped me time when his poops would come
and of course stimulated his bowels to move.
I kept the potty chair in the bathroom, and asked him to sit there, every morning for 5 minutes, NO MORE THAN THAT, I set an egg timer and left he room. no books nothing.
after the timer went off he could come out, even if he didn't poop.
this became a routine for about a week.
Of course he was pooping his cloth diaper and we were flushing.
After about 2 weeks he finally pooped the potty.
and about 2 weeks after that we transitioned to big potty.
Now he sleeps thru the night with out a diaper and never has accidents.
Other things I did
I scheduled play dates with children whom had an older sibling, so that he could see them using the potty and this really worked well,
( 5-6 year olds are the best a t teaching the potty)
we watched the potty video every 3 days or so
I tried no to put too much pressure on him.
One thing I have not mananged to figure out is how to get them to wipe , or wipe well LOL.
Email if you wanna talk
M