If you landed as a PR and your son was already born, I hope that you had declared him as your son and had him medically examined. If you didn't do that before becoming a PR, you can not sponsor him.
If your son was born after you became a PR, or before but was declared and medically examined before you landed, then you can fill out an outland application for him. Depending on who has custody now, you will need to have a paper from the mother ( if she has custody) that she authorises him to come to Canada etc.
Good luck,
Sweden