I am currently living with my sister. My nephew has had encopresis and is still pooping his underwear. While he may have some actual accidents sometimes, it's not nearly as likely because he's been on a regular laxative schedule for a couple months and has long streaks of time (up to two weeks) accident-free. The accident-free times coincide with whatever newest bribe system my overwhelmed and desperate sister has come up with. Both my sister and I are understanding of an actual accident because it's part and parcel of encopresis, but it can be hard to tell because he'll also say he was just too busy playing to go to the bathroom. My personal belief (and I know I could be wrong) is that the physical problem has cleared up enough that he does know it's coming, he's just in a really entrenched behavior habit.
Here's the real problem: he doesn't seem to care. He'll sit at breakfast or play with legos and poop in his underwear and then just leave it there until he's jumping around and some falls out and one of us makes him go clean it up. Often, he'll take off his dirty underwear and hide it so he doesn't have to deal with it and then we find it days or even weeks later. My sister has tried taking away toys, telling him he can't go play with friends, rewarding him for pooping on the toilet, and having long talks as well as using www.ucanpooptoo.com. She's in support groups, but she's at her wit's end.
We have also tried keeping him in the bathroom all day when he poops and doesn't clean it up right away because it's a health and welfare issue (my sister watches two little girls during the day and has another, younger son as well as my own son to deal with) and we can't just have poop flying around. He has toys and gets exercise breaks and can earn back rooms in the house day by day if he goes without an accident. But yesterday he pooped in his underwear while he was in the bathroom. And didn't stop to clean it up. Then he did it two more times.
Using pullups only makes it worse because then he doesn't even care if he pees in them. And really, my sister is way too stressed out to just change his diapers like a baby. He doesn't have any diagnosis. A psychiatrist saw him for 20 minutes when he was calm and said he was fine. I've seen him in action and I'm willing to bet good money that he is struggling with ADD and is almost definitely on the autism spectrum. Neither of us wants to use shame or bribery, but nothing is motivating him. My sister is dedicated to gentle parenting, but she also has a hard time not yelling when she's frustrated. I am willing to take on an intensive approach to get him to pay attention (some kind of NLP program, maybe?) because I have more time on my hands while I'm looking for work, but I'm not sure what to do yet.
TLDR: Need help motivating a 6 year old boy to care about not having poop in his underwear, regardless of whether it was an accident. He has a history of encopresis and we've tried everything.
ETA: she can't put him in school because he's not potty-trained, and she is trying to homeschool him but lessons are constantly interrupted by him having to go clean out his underwear and take a shower.