Child incontinence

[Tom2021]

I have a 9 year old nephew who has for the past 6 years been unable to control his bowel movements.  He regularly goes to the toilet,  however he has an uncontrolled BM virtually every day.  His parents have tried to get help,  but thus far they have been unsuccessful in finding the problem.  We did notice a few years ago that his one leg is very much under developed compared to the other.  I  am not sure if this is related, if there is possibly some nerve issues?