My son potty trained at 2.5 as well. One Friday he asked for underwear, and by that Sunday he had no accidents and even used our church bathroom just fine. Never any daytime accidents at all after that weekend in fact.
BUT- we did put him in a pull-up at nap time and that is when he would poop. He would then tell us and we'd go to the potty and flush it down and clean him up. (I should mention that he quit napping at 2, and that nap time was really just play-quietly-in-your-room-and-give-mommy-a-break time by then.) It took about three weeks of being pee trained before he pooped on the potty.
That whole three weeks he really wanted to poop on the big potty. We had bribed him with a chocolate covered raisin, a real treat to him. And he wanted to use the special Cars potty wipes we got him. (Just plain flushable wet wipes that Huggies sells in a Cars container. We refilled it with regular adult flushable wipes after. Much quicker to use wet wipes than TP on the kiddo at first, which I think makes it less traumatic!)
Still, even though he really wanted to poop on the potty and talked about it, he'd get too excited/nervous when the time came. He was just more relaxed in the Pull-Up. He did fight me about putting it on too, because he only wanted to wear underwear, but I told him he'd wear them for sleeping like his big cousin does.
Anyway, it took a few more weeks till he was relaxed enough to do it on the real potty. Part of it might be that he had privacy in his room and not the bathroom. Even before potty training, he was always a kid to "sneak off" to his room or behind the couch and poop.
I don't know if you'll feel like giving him permission to go in a Pull-Up will undermine the potty training. You know your kiddo. But it didn't really cause a problem for mine. I would be concerned that his first poop in 5 days will be hard coming out and scare him from the potty. I also read that pears and raisins are good for pooping, and that too much cheese, dairy, and apples can add to constipation. Good luck!
