Hi A.!
I can totally relate to your problem! I am a mom of 3, 2 girls and one boy. My son had the same issue as yours. He is now 5, and I promise, he goes # 2 on the potty!LOL! He was afraid to do # 2 in the potty for some time. He did great w/ # 1 like your son. Im not sure what the fear is, I cannot figure that one out, but I can give you some advice. I got so tired of the pull ups, so I just put underwear on him, and if he went #2 in his undies, o-well! Let me tell you, he picked it up quick, because it feels "yucky" to have poop in your pants! I know it sounds cruel, and mean, but it didn't take long. It is gross, but it was the answer for us. And my son would hold it too, but dont worry, when they have to go bad enough, they will! Just dont give in to the pull up. That is just going to prolong the whole experience. I wish you luck!
T.