The Sims 4 & Steam I need my 20 years of progress back

Justthediscoman​  What this sounds like is that installing the game again through Steam wiped out or overwrote your Electronic Arts/The Sims 4 folder.  It's shouldn't have but evidently it did.  But you need to make sure of that.  So, the quick way to check that is to start the game.  At the main menu, hit the Load button and look at the list of saves.  If the only one that's there is your son's, then everything you had is gone.  And if you do not have a backup save that was stored elsewhere on your computer, it's time to start over from scratch.  Hopefully, you have one.  

But when you check the saves and find older saves there, then you can use one of those to pick back up your play.  You might lose a little play depending on when you last played it though.  Better than nothing.

Now one other thing it could be though is what computer account you used for your son on your computer.  My comments above are all based on the both of you using the same computer account rather than separate ones.  But if your son was on a separate account, the game's save files should be in a separate My Documents folder under his username.  Same for you.  So, the question is:  was your son on your account or his own and did you sign in the last time on his account?

Hope this helps.