You have to watch for people trying to re-sell titles - The vast majority of games these days come with CD-Keys (serial keys), and they have to be registered on a digital distribution platform (Steam, Origin, Games for Windows Live). Once that is done, it's nigh impossible to get the game on your account. You really have to buy 'New', to be guaranteed a working title.
You should contact Steam and demand that it be transferred to your sons account. But for the sake of a couple of quid, I wouldn't bother. Have a look below:
Supreme Commander 2 PC | Zavvi.com
There it is
new, for £5.95, with free shipping
I urge you to stick around and keep an eye on the
Game Bargains thread, you might run across a couple of (very cheap) gems
Just to be sure that the machine is capable of certain titles, what kind of spec does your kids machine have? Download this handy little tool -
Piriform Speccy
All you need to do is download it, run it, highlight the 'Summary' page, copy it then paste the results in here.
It's useful to know what hardware is inside the rig, especially before you go and buy titles that your hardware cannot run!