Interesting reading Keith. Going digital seems to be the way forward, unless your in a bad phone signal area, and loose power to the house. Atleast the old copper cable just relied on being connected to the person you where talking too.
Another option as it's your son, you could keep the service active at your current address, your son would have no break in service or need to renumber the line, just change the bank details so the payments come from his account, you'd still be the account holder and liable but the monthly payments would come from his bank account, as a service user he'd still be able to report faults and discuss most things to do with support.
You could then open a second account for your new address.
That's also an option Richie.
I think the best way forward will be for him too start a new account, and take our chances on renumbering. Having talked it over with him he say's it won't be the end of the world if he looses the original number.