There is a solution for that: family sharing. Make him his own account, activate it as family on yours, and the only limitation will be that you can’t play the same game at the same time.

Maybe something changed but last time I used family sharing would allow only one active game per account at a single time. But you can't go offline and keep playing while slightly risking an abuse detection. You can do the same, or at least could, with 'free weekend' games and play as long as you don't go online.

Another limitation is that this sharing isn't enabled for all games especially competitive online games (the system was widely abused by hackers to avoid bans).

I don't know how complete this list is but there are some games you can just launch directly:

https://www.pcgamingwiki.com/wiki/The_Big_List_of_DRM-Free_G...

You can in offline mode.