When using the online product, each tax return requires a unique account. If you filed both your and your son's tax returns using the same user ID and password, you overwrote the first return you prepared with the second return and the first return no longer exists.
If you created a separate account for each return, then you must log in with the correct user ID and password for the account you are trying to open. If you have forgotten it, you can recover it using this link: https://myturbotax.intuit.com/account-recovery
