Marcy--Your question reveals a tension between the use of oral medications versus natural food products in affecting “poo”.  The advantage of medications is that they are more accurately measured, manufactured, and tested by common scientific standards.  So when you ask about a natural food product the same standards are not applied and you are dealing with more anecdotal and unique formulations that are hard to generalize from for advice.  Here are some points to consider.

1). I am at odds with the current medical over-use of oral stool softeners which liquefies stools so as to eliminate normal voiding signals which would come from normal well-formed stools knocking at the anal exit.  I share your concerns, but you have to see normal daily stools in order to evaluate any relationship between diet intake and his BMs.

2). Unfortunately, eliminating stool softeners and substituting fiber does not mean dealing with the engrained holding reflex of these children which also distorts his stools.  You need a daily BM, which is normal, to see normal results.  Then and only then can any alterations in diet be judged over the course of a week or two. 

3).  Liquid intake is extremely important which is often overlooked by parents in stool production.  Any increase in fiber should be associated with added and adequate liquid intake. Sufficient liquid intake alone is important.

4).  Too much fiber could actually be a problem with too much stool production or producing hard and painful stools.  This is too often overlooked in the email forums.

5).  The Soiling Solutions protocol assures daily stool production which means early cessation of soiling and a daily look at normal stool production.  This assures that you can better evaluate the effects of any diet changes which would include tapering off of a stool softener and which can be very unique to the individual child.  I just checked and see that you purchased the Clean Kid Manual-IV and are on the SS Forum, so at least you have a good start.

6). I personally think that the emphasis on diet and possible food allergies are way overdone by parents of children with encopresis.  Why pick fights with the child and the rest of the family over food if the SS protocol alone solves the problem?  The SS approach will work in the vast majority of cases because you are really training in the appropriate voiding reflex to take over from an overlearned holding reflex regardless of diet!  Look at the general population with all kinds of diets and no issues with soiling. I think diet should be based on general health considerations rather than this specific condition of encopresis distorting food choices.

I hope that I have lent some perspective to your question, even though I could not give you a formal simple answer!