Your son may have had his return filed with a different User ID then you are using to log in now or if you used the same TurboTax Online account to file multiple returns, then only the last one filed each year will show, as all other would have been overwritten. You can get a list of your User ID's and recover accounts, by using the tool at the link below. If you have more than 1 email address, you should run the tool on each one.
NOTE:
Before running the tool, log out of all Intuit
accounts including this user forum, or you could end up in a loop. Then
clear your browser Internet cache, close your browser, then reopen it, and go
to:
https://myturbotax.intuit.com/account-recovery/