hi sphil - guess what - SNAP!
dd1 is doing brilliantly for wees - not independently going yet as she can't dress/undress, but asking on time, performing on time etc - but poos are a long way off.
Thankfully she only really goes in the morning, so still has her night nappy on, but we face the same problem - she only ever goes standing up.
She was the same for wees, though, and has (only recently) cracked it. Our vb consultant (who obviously multi-tasks ) reckons her issues with weeing were to do with low muscle tone - she just didn't know which muscles to use to push with, and this will be the same for poo.
Am I right in thinking (sorry for tmi) that your ds's poos are soft/mushy? vb consultant reckoned again that was somethign to do with it - dd1 has never had to "try" to poo - never been constipated or anyhting - she just goes with the flow so to speak, and so does not know "how to poo" when she needs to. It was the same for her weeing - she would do everything perfectly - ask, sit nicely, clearly need to go, but not know how to go (she also only ever used to wee standing up - not so convenient for girls ) but she has recently sorted that out. dd1 was the same re: school and accidents (and also when we were out shopping) which was why we thought her problem was about knowing how to let go - she has never had an accident outside of the house, however long we are out for.
So, after a lot of waffle, my conclusion is that it too will come in time. vb consultant did say he would try to look up someone who might be able to help with the muscle tone bit (the mind boggles really, but there must be someone who would know how to train the right muscles for "pushing") - if he does get back to me, I'll let you know (but might have to chase as the urgency has gone for us for now)
wrt books - I saw the everyone poos book at ddd1's school the other week - it is pleasantly diverting for a child if they are interested, and I think might work for a child scared of pooing altogether, but not sure it would work for loo refusal particularly - it has pges and pages of lots of animals pooing, and then at the end shows a (drawn) child sitting on a potty/loo but iirc the emphasis is more on the pooing (and that everyone has to) than where it goes (could be wrong though, as only had a brief look)