OMG I am so sorry. Our son does the same thing. He holds it so long it hurts to come out and then we have to use a stool softner to even get him to go.
We have tried the following things, and sometimes it works and sometimes it doesn't. I'm sorry, but I hope some of this helps:
Tablespoon of Dark Karo syrup per day
Offering a piece of chocolate each time he poops (the most recent success)
Letting him run a lot each day (it helps move things along)
Feeding him beans to give him gas (which later moves things along)
Stool Softner
Letting him read a book by himself in the bathroom on the toilet with the door closed
Going back to majorly praising him when he does go