I agree with the previous posts that pushing him to be a "Big boy" which
would also mean not being able to nurse anymore might precipitate strong
resistance.   Many people underestimate how much nursing means and how much it
gives to a preschool child.  Too bad all young children do not get to have
this wonderful stabilizing activity to fortify them through these formative
years.
 BUT withholding stool in a pretty common behavior in this age group.  Many
children, especially boys in my book,  are just too busy to stop what they are
doing to "go potty".    Once they have stopped the natural urge a few times
just because it is inconvenient for them, the stool becomes harder and harder
the longer it is retained. Then when the child finally does have the BM it is
large, very hard and painful to pass, which makes him reluctant and fearful
about having ANOTHER BM, which leads to more withholding, and a vicious cycle.
Although it is should not be used for long, and can decrease nutrient
absorption, one of our local pediatricians recommended giving a child with
this problem a little mineral oil daily. (1or 2 Tablespoons--mix with a little
juice and drink down quickly)  That lubricates everything and makes it
difficult for the child to withhold the stool.  After a few regular BMs and it
is no longer painful, the mineral oil can be stopped.  It is a pretty easy,
low stress treatment and can prevent this from becoming a long drawn out
process.

Jane Bradshaw RN, BSN, IBCLC
Mother to 2 boys and a girl.  Both the boys did this to some degree.