Lost steam secret answer

Your gonna have to contact their support, sadly a ticketing system which can get aggrivating:

https://support.steampowered.com/kb_article.php?ref=122...

Contacting their support number is pointless.

You can provide Steam Support with some proof of the games purchased by you/ your son. Accompany this with the last 4 digits of the credit card associated with the Steam account.

Once you can establish to Steam that you are in fact the genuine user of the account, Steam will reset the password and send you a temporary password. This has worked for a number of people in the past.

All the best!