Ds has had probelms with this for years. Unfortunately, once they have constipation/impaction/soiling it can take a very long time for the bowel to return tonormal and give the signals to the body that they need to poo.
As the impaction builds, this stretches the bowel and so when a normal poo passes, it does not give the signals that you and I have.
The soiling occurs when softer poo slips past the impaction and then slips out with the child completely unaware due to the lack of sensation.
First step for us was to talk about it not being ds' fault> They obviously have no cntrol over the soiling. The only thing we asked was to tell us when he needed clean underwear/help with washing himself. This hlped his confidence immensely.
We managed to clear the impaction using lactulose but, I think, Movicol is very good. I don't know anything about doses but the advice to keeping going until soft poo starts does make sense.
You may need im to be on movicol for some time, tweaking the does for him to do a daily soft poo.
He will also need to sit on the loo regularly to get his bowel into a routine. My ds used to sit on the loo 20mins after breakfast and 20mins after his tea.
Water is very important and has been the key for ds to keep his bowels openning regualarly and much more improtant than diet for him. He has to drink LOADS of water, much more than many adults and if he does not drink enough, he starts to get bunged up again.
Have you seen the Eric website? They have useful information, leaflets and a good helpline.
Sorry for long post, I'm sure I have missed loads out though!
If you search encopresis, soiling, pooing on here, you will find lots of other useful posts.
Unfortunately, constipation and soiling is quite common and can take some time to improve. Gla dyou sound as though you have a supportive medical bod to help!