frazzle - my ds did this for ages,starting around age 7....only really moved forward this summer.He will be 10 in January.
I really sympathize..he would do just that - sit in crusty pants,stinking,his 3 sisters complaining,and deny it - sometimes,if confronted with evidence,he'd say it wasn't him!
He would get upset if teased and if I got cross.
I took him to the GP.There was nothing physically wrong and he wasn't constipated.
He didn't want to talk about it.Star charts were not much help.
I think he really wasn't sure at age 7 why/what he thought about it all.What he says now may be of some help......
he didn't like being told he was smelly much, but he says he didn't really care much if he was dirty...didn't really notice.He hated having to get cleaned up - didn't like to put his fingers in it to wash,but didn't mind it being in his pants.
He became immune to his sisters remarks and my frustration and annoyance - tuned us out.
He now says there were two things causing this.Firstly,he hated going to use the school loo for a poo as other boys peeped under the door,and because there was no soap to wash his hands (!),so he got good at "holding it"and waiting to go at home.But he found it would begin to leak out when he ran around at breaktime.
He would come home and go,but by then his pants were already dirty...sometimes he would just let the rest go - if he was busy with more interesting stuff he decided he would be moaned up and have to wash anyway,so in for a penny,in for a pound.....
Which is really the second reason..he says he just couldn't be bothered enough,and became immune to the consequences - whether understanding or cross...too many distractions and a reluctance to engage with personal hygiene...
Well,we have gone the whole summer now with just an occasional quite whisper in his ear that a whiff was apparent and he has really tuned in to it now.He used to hide his pants down the side of his bed,but his dad came round during the holidays to help him re organise his bedroom and ds was very embarrassed when his little stash was discovered.
Hmmm,maybe this isn't too helpful,as it seems to me that it has just got better over time,really...
I know poo witholding can be related to a fear of letting go...of strong emotion...well my ds has had a bit of an anger issue lately,(which is hopefully now settled,too- posted on NPD thread in Relationships) so maybe that also had a part to play.
Good luck!