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.