Please remember that any mention of competitors, hinting at competitors or offering to provide details of competitors will result in an account suspension. The full rules can be found under the 'Terms and Rules' link in the bottom right corner of your screen. Just don't mention competitors in any way, shape or form and you'll be OK.
What do you have already, have you got a computer at all?, or are you building one from scratch.
Supreme Commander can only use 1 core, but requires a large amount of processing power, therefore a Haswell i3 would be the best option for playing this game, in terms of GPU nothing super powerful is required, hell I could even run it on a AMD HD 5750; So you might even be able to run it off the integrated graphics in the i3 (HD4400).
Also due to the way this game was made, it runs at the speed of the slowest computer in multiplayer, so if your son's computer is not that fast, there is no need for you to get a faster computer.
And another thing, due to the age of the game the multiplayer server where shut down a few years ago, however a person in the community built a replacement server, it has a strong following and has expanded upon the base game quite a lot, even adding a new race!
It's called forge alliance forever or FAF: http://www.faforever.com/
But back to the point, if you want to play this game you need a Intel processor that's dual core (as it only uses 1 core), RAM wise its a 32bit game so it can only use about 3.5 gig of RAM so you could mange on 4GB of system RAM, Graphics Card wise, I'm not so sure, and I will check on the weekend, but anything of mid range should run this more than fine at max settings.
So to sum up, if you could post what you have at the moment(if you need a full computer or just upgrading one you have) and what your sons computer is, I should be able to help more.
Any other questions just ask.
Supreme Commander is multicore and will chew as many up as you have. You can even download an enhancer which will also make better use of as many cores as you have. See the wiki.