Is it possible to split a Steam account?

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:

Why not just give him his own Windows login, and put shortcuts to the games for him on his desktop? You can remove the Store option from the taskbar icon; and remove it from the Start option too. I would also encourage keeping the computer in a supervised area, and not just because of Steam.

Unfortunately, I don't think so. That's the cost of doing business digitally I'm afraid -- Steam doesn't allow refunds or game transfers.

The one thing you can try is by contacting Steam support, as outlined in this thread, but as you may imagine, chances of success are slim.

Though I'd love to be proven wrong.