Hi R.,
I just wanted to let you know that I feel your pain. My son is in the exact same boat right now and we are constantly working through this. We seem to have good weeks and bad weeks. Last week was great, but this week is bad. Here are some of the things we're doing that are working a bit (and I'm not above bribery on this issue):
1) Potty chart just for pooping that leads to a long term goal. For my son, a very expensive pack of underwear is what he wants, but we've told him he can't have them until he's consistently pooping on the potty.
2) His ped. recommended putting a diaper on just for pooping in case he's scared of the popping in the potty. Unfortunately, my son refuses.
3) His ped. also recommended using mineral oil to help soften the stools at 1/2 tsp to 1 tsp a day. We haven't tried this yet.
4) Here's the one thats working best right now: bribery. We have a stash of inexpensive toys in our closet that we will put out on a shelf for him to see, but he can't have unless he produces a (decent sized) poop in the toilet. He'll sit on the potty for marathon potty sessions and he can hold the toy while he's on the potty to help motivate him, but he can't really have it til he's gone. Whatever toy that your son loves and will be a big motivator works. For my son, its legos, but we've also used bubbles, sand box toys, puzzles, books, magazines. Sometimes, its a complete surprise and sometimes (when I know he's really resisting going, I let him pick which one he wants and it will go on the bathroom shelf. We've started fazing out toys for each poop and now we're using different motivators, like a trip to someplace special he likes or extra bedtime story.
I hope this helps. I'll keep checking this thread, but please email me if you get some amazing sage advice. Good luck!
-H.