Oh my word! - This could have been my post on many boards about 8 months ago. Only my son was almost 7 and we had never managed to even get a little bit of poop in the toilet. Now granted my son has developmental delays so real-age-wise he wasn't really 6, but he did all the go hide, can't do it in the potty bit. I was resigned to changing diapers forever and then in January, shortly after his birthday, I started something new. I made "sitting" part of his routine. I'd have him stand to pee, then set the timer and he'd sit for 5 minutes. He got a treat if he sat without fussing and we didn't make an issue at all about poop. When he had messes, I'd clean him up with no scolding, but would say, it's OK, when you're a big boy, you an do it in the toilet. After about 3 weeks of that, he actually produced in the toilet and we made a big deal of it, clapping and cheering. Within a month after that he was regularly producing several times a day - pretty much on demand. And now we rarely have an accident - he even tells us when he has to go!
Long story, but my point is, he had to be developmentally ready and so does your son. Take the pressure off by practicing "sitting" instead of focusing on "pooping", no fuss when he does go in his diaper or underwear, and see where it goes. I had to get to the point where I understood that in the larger scheme of things, spending 15 minutes a day leaning up messes was nothing compared to what some people go through with special needs kids. And for you, enjoy all the cute, sweet things your son does, make hanging the messy diapers a non-event and know that, this too, will pass!