Hey @Add_56841, Welcome to the 3Community 🙂
Are you looking to add a prepay or a bill pay / SIM Only Sim Card onto the account and I can help you here with the process.
When you are on the Dashboard of your My3 account you should see the option to "Add New Account" at the top of the screen. Once you select this you will be able to order a new number on your account. For your son the "Three Total Talker" would be my recommendation as this prepay plan comes with unlimited calls and texts.