Steam – Do the Steam ToS allow a single person to have 2 Steam accounts

Unfortunately, Valve will not let you transfer games to a different account, unless they are gift purchases. As suggested in SqlACID's answer, you can create a separate account login for your son that still uses your Steam account login and password.

If you have Windows Vista, Windows 7 or Mac OSX you can additionally setup parental controls to disallow the launching of games that you don't want him to play.

If you want him to be able to launch games directly from Steam, I would suggest creating a new game category specifically for him and then add all the games you want him to have access to. To do this, right click a game and select Set Category..., then add a name. You can then select that category at the top of the games list. Here's a rudimentary example:

Because of the way that steam does things it's not able to have multiple auto sign in configurations per installation. You'll have to resort to trickery:

• Disable account credential saving in the settings dialog.
• Create a shortcut for Steam on each windows account, edit the properties and give it these launch options -login %u %p. This logs into Steam with the specified Username and Password combination. Replace %u with the username, and %p with the password you want to login with (Steam must be off for this to work).

Source.

If you still want Steam to automatically startup, while logging in to the appropriate account, disable automatic startup within Steam and place the shortcuts in the Start menu → Programs → Startup folders of each windows account.