Get rid of the Pull-Ups.
He needs to get comfortable using the toilet to poop. Taking away the Pull-Up is a start. My experience with children is that Pull-ups are confusing: kids think they are some kind of magic underwear and *not* a diaper. Being very clear that he either needs a diaper or to use underwear will help immensely. (With our son, we gave the choice of cloth diapers or underpants. He chose the underwear!) The feeling of defecating into real underwear, and the mess it makes, is actually very instructional to children. Yes, it is a big gross mess to clean up. *However*, it is more helpful to them to feel the big gross mess than to not feel anything other than "a bit of poo comes out".
This is, ultimately, his challenge to deal with. If it were me, I'd offer foods with plenty of roughage, so he's less likely to be able to self-constipate. I'd also suggest checking through the list of assessment questions on this site to see if there are any missing pieces of the puzzle.
http://www.betterkidcare.psu.edu/AngelUnits/OneHour/Toile...
There's more information, too, which can give you some guidance and encouragement.
Using the toilet for #2 is a pretty common stumbling block. Please do not pressure your son about this with preschool, either. He sounds like he's already a bit frustrated, as are you, I know. But the pressure of not being able to do something 9 months from now (which is FOREVER for a child) is just confusing to him and distracting from what you want him to do: relax.
Ina May Gaskin has a great book called "Guide to Childbirth". One of the concepts she explores is what she calls "sphincter logic", which suggests that when we relax our bodies, we allow our natural tendencies to birth to work better. She even has some techniques for this in the book, in the appendices, so if you can check it out from your library, please do so. I found it revelatory, and used these methods birthing my son. Along with an absurdly short labor for a first child and quick dilation, I also used these methods in the days *afterward*, when (ahem!) everything was painful coming out. They helped immensely, or I wouldn't even be writing about this.
All this to say, helping him relax is important too. Give him some time... this is a very common challenge for young children using the toilet.
Best of luck,
H.