Right click on Civ5 in your list of Steam games and select properties. There's an option to disable auto updating for each game individually. There is no hotseat mode, you can however have it installed on one computer and put it into offline mode and have it on another and LAN them. (I haven't tested this as I don't have Civ5 but I know it works with other games). And sorry, but the game is now unalterably linked to your son's Steam account. You can only have the account online on one computer at a time (but you can log on, go offline, then go online on another computer). Happy birthday to him eh? You could get him to buy the game for you and make sure you're logged into your own or no account when you install it next time. Kind of crappy but I wouldn't blame Steam for assuming the account that was logged in was the one you wanted to install the game too.