I am assuming this is the BT Premium Phone with 100% nuisance call blocking and mobile sync. If your son calls you what do you see in the call history. you should have the option to save this as a contact and as a VIP. You can also Store 1000 numbers on your Premium contact list, by synced mobile phones (you can register two mobiles at any one time)
If that fails call the BT helpdesk on 0800 145 6789, they are open Monday to Friday 9.00am to 5.30pm, Saturday 9am to 2pm