Mason is calling on the Child and Youth Advocate to conduct an investigation into the death.
Mason says there have been 15 deaths of children in government care in the last four years.
He says there needs to be "an investigation that produces a report that the public can see, not one that’s hidden by the Conservatives.”
The boy was spotted lying face down in the pool.
Lifeguards pulled him out and tried to resuscitate him but failed.
City officials say they are launching an investigation into the death to see if there are any steps they can take to improve safety at their facilities.
Spokesman Rob Smyth says it appears that the proper number of lifeguards were in place and that other steps to ensure the safety of pool users were followed.