My 12 yr old has a health condition this means he doesn't absorb fats and vitamins which leads to chronic diarrhoea amongst other things.
This can be controlled with drugs very easily if he takes them before food.
However he just cannot seem to do it, if I hand them to him he'll take them but if left to take them himself at school (just staters highschool) or if I say 'can you take x amount of capsules before starting dinner' he'll go into the kitchen and come out saying he's done it.
Several times in the past few months he's soiled himself due to not taking this medication, once was was very close call as it happened when walking home from the station after school, of it had been a few mins earlier he would have been surrounded by friends when it happened. I had a chat (after coming into an awful smell and poo covering both bathrooms and his underwear discarded in the bin, he'd got changed and gone out)
We talked about how if this happens at school it will be really embarrassing for him and that it seems silly to let it happen when taking the pills is so quick and easy.
I also said that I'd leave cleaning stuff in each bathroom and if not did happen could he clean up after himself properly and put his underwear in the wash rather than leaving it in the bin.
All this was totally non judgmental and discreet.
I thought we'd got through it but this last 2 weeks I've found 2 more pairs of soiled boxers in the bathroom bin and today another set in the wash and I've walked into the bathroom this morning to find it covered in poo and I stood in some. 
So now not only is it likely he's gone to school smelling of poo, he's obviously not been taking his medication despite telling us he has.
I'm really worried this will happen at school and he'll be bullied for it, I'm also at a loss of how to proceed, he won't engage with the dietician or the psychologist about it and when asked how his bowels have been he grunts and shrugs and says fine, if I say different he denies it.
I'm also incredibly fed up of mopping up after a 12 yr old who is mature enough to take himself on the train to school etc but can't manage to clean up after himself of let someone know what's going on.