No such thing in Steam

You'll have to create an account and let him use it.

Technically this breaks the Subscription terms unless you get explicit permission from Valve.

Its also worth noting SFS is awful for families as it blocks concurrent use of different game from 1 account. So if you play anything they can't, even if its an entirely different game.

Steam isn't designed for families, and Valve is adamant that it won't ever allow concurrent use.

If you are a family of gamers, every other platform is far better.

Personally I use gog.com