It is not unusual for a baby at this age to stool infrequently.  i think
ther explosive bit is a separate issue dealing with the sensitivity to
milk/soy.  My last baby started to stool about every 10th day from age 7
weeks to 7 mo.  Of course that 10th day was a major mess day.  In between
it was lovely, no dirty diapers!  I also noted that he was plump, growing
rapidly.  Other babies I have seen like this are similar-growing rapidly.
My theory is that they are using the breast milk so efficiently that there
isn't much waste, hence not much poop.  Sincerely, Pat in SNJ