I would agree that you and your boss don't understand encryption.

someone has to see the account number, who is it? And when? Can you change the code that allows them to query the account number?

If you just are trying to prevent them from seeing the data, you can use grant/revoke. Remove all rights to the table for all users.

Create a view, and grant rights to the view

CREATE VIEW ClientView

as

select ClientID, ClientName, '#$FW$FEDEED' as clientAccountID'

from Client