You'll need to follow the same process as when you joined except using a different email address and account logon for your son.
Unlike some other mobile providers 48 don't support family/group accounts allowing multiple SIMs/memberships to be managed using a single login.
Every user/SIM/phone number needs a separate My48 account and login (email address/password).
See the New to 48 FAQs for details on how to get a new use set up on 48.
If you need to contact 48 support about this then see the links in my signature below.
