I know what you are saying. I had the same problem with my son. He was 3 when I started potty training him. He was trained to Pee in the toilet relatively easy but he refused to go poop on the toilet. To the point that he would sob and sob.
I was using pull ups at night but just underwear during the day and he would wait until bedtime, when I put his pull ups on, to go poop. Then he would just go in his pull up.
It didn't help that my FIL (who watches him while we are at work) encouraged him to go in his pull ups. He would say..."oh, you have to go poops?...Here, and he would put a pull up on him and say, ok, now go."
So...what I did was...I bought a child's potty seat. Its a little small but did the trick for us. I brought it in to where a TV was and I allowed him to watch TV while he was sitting on the potty trying to go. After a while of doing that he started to go regularly in the potty seat.
Then, after he had that mastered. I bought a kids toilet seat cover that I found at
Walmart. It is a white padded seat with a hard plastic bottom. It just rests on the regular toilet seat. I got that because his butt was too small for our normal toilet seats and this closed up the gap to allow him to sit comfortably.
Of course, at every stage, when he actually went poop as he was supposed to, I praised him and made a SUPER big deal about how wonderful it was that he is going poop on the potty. Big hugs and smiles...etc.
Don't yell at him, no matter how frustrating it gets. I know....I spent about 6 months trying to get him to go before I finally did the potty chair in front of the TV thing. The TV just distracted him and let him relax, if you don't allow your kids to watch TV, then a DVD movie would be fine too. I tried reading him a book while he was on the potty but that didn't help my son.
The good news is that once he started catching on...it was a quick transition for us and he figured it out quick. He will be 4 in November and is now completely potty trained...even at night.
I did have some soiled underwear days but really, it seemed like that phase didn't last long. Cleaning the potty chair out is gross but in the end it was worth it to get to the prized "going in the regular toilet like a big boy".
Now, the only help I have to give him is to lift him onto the toilet and to wipe his butt.
My son is going into preschool in September and I hear ya about the stress of getting him trained before then. That was my ultimate goal...I just accomplished it about a month early. PHEW!!!
You can do it...and your son WILL catch on. I know it seems right now like he will NEVER go to the bathroom in the toilet but he will. And then you will look back and say, "did we actually go through all that?????"