S.H.
This is the age in which this happens. It's common.
It happened to my girl at this age too, and we had to take her to a Pediatric Gastroenterologist, because she became greatly constipated and her BM's were impacted, inside.
Main thing is, he does not get constipated. It can trigger a vicious cycle if they do... because, per our Doctor, it causes 1) pain 2) then they don't want to poop 3) then they get more constipated 4) then they get more pain, and on and on.
This "stage" is partly emotional/psychological and physical.
The more they hold it in.... the more it can 'hurt' them....then the more they don't want to go poop.
ALSO, holding in BM can also cause problems internally. So keep that in mind.
Here are some links on it:
http://en.allexperts.com/q/Pediatrics-1429/Struggles-hold...
http://www.drgreene.com/21_762.html
http://www.medhelp.org/posts/show/278442
I would take him to the Doctor.
Take care,
Susan