Lots of kids feel this way. My son wasn't reliably potty trained even for pee until well past the age of 4. I honestly don't think you can do much more. If rewards don't work, leave it alone. It becomes so so stressful for the child because he wants to please you and it becomes a question of shame. That's so hard to work through.
It's a developmental thing. Assuming he's not uncomfortable (and many kids are because they don't eat the right foods to prevent milk constipation which makes pooping so annoying for them), I'd back off. You're frustrated of course, but honestly, you can't do anything but wait it out. He's doing so many other things on or ahead "of schedule" - celebrate those and let him get to the potty thing when he's ready. Put him back in pull-ups and stop the discussions and the underwear accidents. I know that's not what you want to hear but truly it's the only way.