Don't push it. He's still not ready. I went through this with my five year old son, and now It's the same thing with my three year old son. My older son didn't even start peeing in the toilet until he was four. I tried the underwear thing but got tired of cleaning soiled ones, so I did pull ups. Anytime he wanted to pee he went in the toilet, but if he had to poo, he was allowed to do it in his pull up. He knew he should be pooping in the pot, so while he was at pre-school, he wouldn't poop in his pull up, but when he got home he would. One evening after dinner, he was taking a nice warm bath, he told me he had to poop, he remembered an incident when he was three of pooping in the bathtub, and didn't want that again. I told him, well we don't have time to put a pull up on, so hurry up and jump on the toilet seat, he did, went poop, and has been completely potty trained ever since.
He was over 4yrs old when that happened. I just don't think kids should be pushed, or told that they are not suppose to poop in their pull ups. They're training pants, that's what they're for. You don't want them to get impacted bowels or have a constipation problem. I think this is a bigger issue with boys, since most learn to pee standing up, they become afraid to sit on the potty.
My 3.5 year old is now doing the pull ups, he's peeing in the toilet, but not ready to sit and poop on the pot. He would go nuts if I used diapers on him again! Get rid of the diapers, use the pull ups, and give him some time. Good Luck!!