Can users play simultaneously from one copy of Civilization V?

Civilization 5 is designed to work with Valve's Steam platform.

This will allow you to install the game on any number of computers, but requires you to log-in with steam to play. Since only one steam account can be logged in at once this restricts you to playing the game on only one computer simultaneously.

The exception is that you can run Steam in 'offline mode' to play the game exclusively in single player mode, in which case multiple computers could each play 'offline' at the same time. From what I understand it is not possible to play a LAN game this way, you would need two copies of the game (and two steam accounts). Apparently it IS possible to play on LAN in offline mode. If so, this would allow you to play with your son with a single copy of the game.

This would seem to be a deliberate Steam design choice, for better or worse.

I'm not sure why an ad would specify the number 5, since Steam would not limit the number of installations at all, and only one account could play online at a time.

Seeings how it would be tied to your steam account, and your steam account can only be logged into one computer at a time, I do not believe you can play on multiple computers with one installation. Instead, you would need to gift another copy to your son's steam account, and play together in that way. I believe the 5 different computers just refers to the number of machines that can have the game installed on it, not the number of copies that can be played at once.