Anxiety, His Mom went from his life at a very young age when he was five, ask him what he does with his Dad do they have one on one times together ,how does he get on with his Dads new friend, His Dad should chat with him and ask about the Bullying going on,he needs to ask the school to keep an eye on whats going on.Some councelling would help.
By 'case study' do you mean a hypothetical scenario on paper, or are you being sent to work with a real family? I am sure if he is being bullied, and is soiling himself, then his aches and pains are very real and need to be dealt with. I am sure you mean that when he was five years old, his Mother died, and not as you said 'His Mother died at the age of five.' Enuresis, or 'Bed wetting' is not that unusual or uncommon in young boys. My suggestion would be for his Dad to take him to a Doctor. The reasons for his accidents are most likely the confusing changes in his life, and he just has not learned to cope yet, and the bullying is only making matters worse, and if it's not stopped now, the 'teasing' could follow him for most or the rest of his life. It might not be such a bad idea for him to move School if the bullying does not calm down. Every boy and girl needs a hassle free start in life.
Not sure what you mean by 'why'. Didn't you just tell us that he was being bullied at school and now is facing change at home on top of the death of his mother 3 years ago?
Isn't stress and anxiety enough of a reason?
Or did you want something physical, like constipation or a urinary tract infection which might cover the upset stomach too?