Maybe it's the idea of changing a routine that he's so familiar with thats bothering him more than actually using the toilet. Have you asked him what would make him use the toilet instead of his pants? Maybe something that he can have in there every time he poops? I personally like a good book or magazine. Maybe find something like a hand held game that he can use only when he's going to poop. Or if he likes reading, a book or magazine.
Also, it could be impatience. I'm an extremely impatient person, and I hate doing things that take a long time. Especially if I'm just there bored, waiting. If he had something to occupy his mind that may help. I'm never on the toilet long and hardly ever have been, and I'm 47. (I'm the aspie in the family, my husband and kids are NT) I just wait until I really have to go, then I go to the bathroom. I detest sitting on the toilet for a long time. If I have to be there, I want a book. I have never understood how people go to the bathroom to poop and sit in there for 20 minutes or so. That could also be part of the issue.
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I'm giving it another shot. We will see.
My forum is still there and everyone is welcome to come join as well. There is a private women only subforum there if anyone is interested. Also, there is no CAPTCHA.
The link to the forum is
http://www.rightplanet.proboards.com