timandra Your post has me even more worried now!
While I see exactly where you are coming from in terms of allowing DC's to be involved in their own care, and give them some control/autonomy - my DSS is certainly not capable of some of things you suggest - should he be, at 9 years old?
He can't (won't?) wash or shower independently, so although I love sashh's idea - it isn't something he can be left to do at the moment! He seemed quite happy to return to his room and continue playing despite the fact that he had poo over his clothes and hands - he made no attempt to clean up or tell anyone, either!
Given the choice you describe of risking embarrassment of an accident at school or seeking medical treatment, he would undoubtedly ignore the issue, rather than acknowledge that he has some influence over it. He lives his life with things happening to him, and seems to have learnt to accept them, bottling up any reaction or emotion he may feel. As DP doesn't know what it is, there is always the risk that permanent damage could be done to DSS health if he doesn't seek medical treatment until DSS decides he wants to.
He says that he is not in pain, and if that is true, then I don't think that he would have any motivation to change things; he just doesn't seem to realise that he can.
If he did have an accident at school - I am certain that he would stoically tolerate the consequences of being teased and being shown up, without expressing any emotion, accepting that is part of life, a not very nice bit, but something that has to be put up with just like a lot of other parts of his life
He seems to be unaffected by things that you would expect DC's to be disappointed or upset about - not in an unconcerned, oblivious way, but in a resigned, here we go again kind of way.
He is very, very quick to say what he thinks people want to hear in all aspects of his life - even admitting that he has damaged something when he clearly wasn't involved. When DP has spoken to him about it, he says he thinks he should do that because he will get into more trouble if he disagrees with an adult! I think that may have been part of the problem when talking about things to the GP - DP said that the GP was asking leading questions; "do you have diarrhoea every day?" rather than "how often do you have diarrhoea?", so DSS just nodded because that's what he thought the right answer was.
As for writing it down - there is no way he has the understanding or skill to do that!
I've been really careful not to compare his development to my own DD (who is 3 years older) - I know that boys and girls develop at different rates, and if there was concern about his development, the school would have picked up on it, wouldn't they?
I know it sounds like I'm making excuses for him now - maybe that's why I am at the end of my tether, because I am so out of my depth with him! Based on my experience, it does feel sometimes as if he is a much younger child -but he is entitled to the respect and independence that comes with his age, which leaves him without some of the basic practical and emotional support that he seems to need. 