I am of the view that to relinquish your rights over your Son, just a simple declaration will be sufficient executed before the Notary .But to protect yourself in the future, first you have to make up your mind that you will not claim your Son in future before you give your consent to the Biological Certificate for adoption of your Son by the step father, as once you give your consent to the Biological certificate you will lose your right in the future to legally sue for parental rights based on DNA as proof that you are the father.
Now, if you as the biological father is agreeable to this arrangement the adoption attorney there in Canada will first proceed to have your parental rights as biological father to have been voluntarily terminated once you give your consent. Once parental rights of the biological father is terminated then the step father can adopt the child.
I would caution you to think over very seriously before you give your consent, so that everything is done legally and in the interest of your son and yourself which will prevent repercussions in the future should you have a change of heart. People grow and change so dont think it won't happen to you. And once the step father has completed the adoption of your child then his name will appear on the birth certificate etc as his father, and you will have no claim over your Son.