Thanks for your post on our Community Forums @bethbradbury, and a very warm welcome to you!
When a customer is seeking to cancel a contract with us, they would primarily need to be the account holder and then secondarily be able to pass data protection
With the account holder we can also issue a memorable word hint on the account, and secondary data protection questions in relation to the account and the billing on the account
Does your son have access to the bills that are owned on the account? These will likely be a very helpful source for secondary questions for him to pass data protection on 0345 600 0789
Kindest regards,
David_Bn