It wasn't too long ago that I commented on mamasource that I was discouraged by my son's inability to catch on to the whole potty training thing. We put him in pull-ups a little after age 3 but it was nothing for him to do his business in his pull-up. We put him in big boy underpants after his teacher in preschool said he was going all day with no accidents which was when he was a little over 3 1/2. He turned 4 in November and still was pooping, and occasionally peeing, in his pants, but pretty much only at home. I assume this was because they have set potty breaks at school and he would get so caught up in playing at home that he would just forget to go. Also, like your son, he too had no problem with wet or messy underpants.
At his 4 year check-up, I asked the doctor about it and he said to not force it, especially the pooping part. He said it's sometimes "a deep seeded issue" for some kids and when you try to force it, the kid can try to hold it in causing impactions which can be very bad.
My husband and I tried very hard to be patient, keeping him in big boy underpants and changing poopy ones on a daily basis. A couple times, we caught him right before he was going to poop and got him to go to the potty on time. Then, one day not too long after that, he just started going by himself. It's like all of a sudden, he could tell when he needed to pee and poop and we didn't even have to remind him anymore. We did have "big" rewards (2 different Lightening McQueen matchbox type cars) for the first 2 times he pooped on his own and then "benems" (what he calls m&ms) for each time after that for about a week or so. He hasn't had a poop accident in several weeks and he only asks for "benems" on occasion now after he poops. He did pee in his pants just a couple days ago, but I think that was because my husband forgot to dump the potty out from the last time my son pooped (I wasn't home) and he didn't want to use it. After I cleaned it out, he went just fine again.
My point is that all children are different. My daughter trained well before she was 3 and only had a few accidents before she was fully trained. My nephew just turned 3 at the end of October and he's been peeing in the potty for at least a few months. He's still working on he pooping thing, but he tells my sister-in-law when he needs to poop so they can put a diaper on him and then he poops so at least he can tell when he needs to go. My son wasn't even interested until most kids are already trained, but when it clicks, it clicks. Just be patient and encouraging. They will do it in their own time.
Good luck! :-)