apathetic 7 yr old deficating in pants

Thanks to everyone who has replied. We have tried making him wash out his own underwear, it doesn't seem to phase him. He just ends up making a bigger mess and doesn't ever get them clean. We started making him throw them away and use his own money to buy more. He doesn't care one way or the other. He already does chores around the house; he must keep his room clean daily and he does his laundry once or twice a week. He gets other chores to do sporadically, but those two are a regular thing. We have taken him to a psychologist, mainly due to his ADHD, but will be taking him back very soon. According to his pediatrician he's doing it out of laziness. The saddest part is the fact that he will walk around in it. The only way we know it's happened is when we smell it or see it in his laundry basket. Just this past weekend, he went from Sunday morning to Monday night @ 7:00 wearing it. (He was at his grandpa's house & we found it when he got home.) I just don't understand and I can't let it keep happening. I've got to do something about it. He has a little brother that he could make very sick by keeping this up. Any other suggestions would be fantastic!!