My brother soiled himself from around 3 years of age, up until he was around 16 years of age when he had the A.C.E procedure,
https://www.shinecharity.org.uk/spinabifida/aceprocedure
"The Antegrade Continence Enema or ACE is a way of managing bowel emptying at a convenient time each day, and should reduce both constipation and soiling episodes. The procedure involves sitting on a commode or toilet whilst a washout is given through a small opening or stoma on the abdomen."
He finally had it reversed after about 3-4 years of treatment, and now functions normally - in the bowel department.
We initially thought it was laziness, causing severe constipation and leaking, however I now after a lot of research believe it's caused by ADHD, all due to an answer to another post I found
http://www.medhelp.org/posts/Child-Behavior/Why-is-my-10-year-old-soiling-his-pants/show/2734788#post_answer_header
The link in the answer on the post didn't work so I took to google and found this,
http://www.webmd.com/add-adhd/childhood-adhd/news/20131021/kids-with-adhd-often-prone-to-bowel-problems-study#1
I was diagnosed with ADHD on 29th of January 2016 just before my 29th birthday in March, I'm the younger sister, by 2 years, I believe my brother has undiagnosed ADHD, I have also been diagnosed with Autism, as has my nephew, and Hyperactivity and Autism have the same cause, as per what I found here,
http://www.youramazingbrain.org/Res_1st_Angelica_Ronald.pdf
So if you haven't already looked at the possibility, that the child/adolescent soiling themselves has ADHD, I think you should check this out, it may help :)