You follow the normal steps for getting started on 48.
If you're already with 48 yourself then you need to use a different email address and login for your son's SIM/account.
On 48 every SIM/account requires a separate email address/login and there is no "family/group" account option to manage more than one SIM through a single login.
To get the best deal you should use the Recruit and Reward option when ordering the new SIM:
If you need to contact 48 support about this then see the links in my signature below.
