I'm a bit unclear what you wish to do exactly, but let me assume you want to use your new phone with your own number and plan, and give your old phone to your son with a prepaid card and his own number. Did I assume correctly?
In that case, take the SIM card out of your old phone, and put in your own new one...
Then take the prepaid card, activate it with a new number and put that in your soon-to-be-your-sons phone.
If you meant something else, please explain what you'd like to do :)