If one of those games is offline, you could disconnect that machine front the internet and start Steam in offline mode.

This is what I do with Skyrim, but sometimes I needed to install or update something. It sucks you can't create a family account and transfer/gift games to that account(they can limit to 2 family accounts), the family support they have is very limited , last time I checked a family member can't play one of your games while you want to play something else.

You mean when he is using your account (or family sharing) on the other machine?

When using my account. I research family sharing and it won't help in this case either, it would let you "transfer" the game but unless you go online. This was 1 year back and I did not research again but I think they did not fixed it to I can permanently transfer games to a family member.