Cheapy,
Do you know about this project? - http://detroitneedsrobocop.com/
They raised $67k to build a statue of Robocop in Detroit! What are the chances you will make a pilgrimage when it's completed?
Yes, do exactly as you speculated. Buy as gift and enter your email addresss. Then your son can click the hyperlink while he is logged into steam. Steam will popup a window with a button that says, "Open my gift." Click this button, and it will tie the game to his account.
I'll second the opinion on Trine. However, I would recommend a controller (Xbox 360 if you have it) over the mouse & keyboard controls of Trine. It really gives you that classic platformer feel. The music/story/narration is very good.