I completely agree with the above. Any 11 year old who is trying to order diapers for himself is crying out for help. This is way different then the other posts we get where the child is seemingly indifferent to what is going on. Here is a link to a very good article that may help you ; http://www.practicalgastro.com/pdf/November02/LoeningBauckeArticle.pdf
And definitely get him into his doctor so he can be checked out. Keep us posted. Good Luck.
-- (I'm pretty sure they make pull-ups in pretty large sizes for kids.)
Please look up encopresis. If this sounds like what he is going through, show him the information and talk about it. At 11, he is old enough to be actively managing his own health care with you, and it will help him a lot (psychologically) to know this is not his fault. Get him to his pediatrician and if the pediatrician does not know much about encopresis, ask for a referral to a pediatric gastroenterologist. In the meantime, once you and your son talk about it as a medical problem to be solved, and come to the understanding that it is time to see the doc, by all means don't withhold pull-ups. The poor kid is trying to manage an out-of-control bowel almost by himself, and he could use an ally.
No! You need to have your son evaluated by his pediatrician to rule out medical issues that could be contributing to these behaviors.