Hi C.- if he doesn't mind pooping in his diaper, he may not be ready to go on the potty. This can be really frustrating and turn into a huge power struggle, so just let it go.
I waited until almost 3.5 with my youngest boy and until all the signs were there. Peeing was pretty quick (after a bunch of only would go outside, then outside in the potty seat..) but pooping another story entirely. I had him in underpants and he would NOT poop on the potty but really wanted to but something was upsetting him about it. After an accident where he was really sad I decided to just forget about it and try again in a month or so. (Turns out he was afraid to sit on the pot- he actually thought he could get flushed down with the H2O.)
Well, when I went to put him in a diaper next day after the poopy pants incident he wasn't having it. Did NOT want to go back in a diaper. So I decided, well, you are on my program now. :)
We then did a 5 poop sticker page with Thomas train where if he did 5 in the pot he could pick out a train. I figured by 5 times it would be a habit and sure enough... worked great. But I waited for him to want to and worked with him on this... didn't rush anything.
So hang in there. Give him space. Ask if he looks like he needs to poop and if he doesn't want to go on the pot, well, maybe he will next time. Offer a diaper for him to poop in or have him back in diapers until he's ready to do both on the potty. And if he wants to stay in underpants, motivate with a fave toy or book or something for 'X' times on the pot. He'll get it- they all eventually do.