.... What?
If you have Steam installed on any number of PC all you have to do is access with your credentials.
Pretty much like any other software that requires authentication.
I- anyway- suggest another thing. YHou can share your library (games) with your son and vice versa.
In this case you see both account games as available to you and you can install and play them.
The true owner of the game still has "ownership rights" on the games so if your son is playing one of your games, and you start that exact game, you can play immediately and he has three minutes to gracefully save&quit.
I love this- thanks to my two brothers interests in different kind of games our combined Library reached over 800 games.
The world is mine.
I am the gaming God.