Here:
http://www.cic.gc.ca/english/pdf/pub/custodian-parent.pdf is a form to appoint a custodian for a minor child. Basically this is for a non PR child and I found it in the study permit section but it should work for you. It's basically because without you there, if there is any problem or emergency, somebody needs to be responsible for your son. The aunts will do fine, according to the form, the custodian only needs to be 19 or older. After you get it signed, you also need it notarized.