If the doctor has told you that there is nothing wrong and tested him then it probably is behavioral and some more counseling will help, does he seem upset by anything you say he doesn't like school is there any bullying .?
If there is some impacted bowel, he can get encopresis and it bypasses the impacted stool and just slips out without him being aware of it. I'd double check that before blaming him or thinking it is behavioral. Check impacted bowel on Wikipedia. It can take up to a year to treat it once the impacted stool is removed.
Now I want to ask you what your definition of soiling is ....mine goes like this ...the child isnt wiping properly so his underwear is always 'soiled' now I ask you is it more than that is he having a 'full' poop.??