First off ,you need to specify a budget. This project could range anywhere from 400$ (if just an upgrade) to well on to 1500$ or above (if getting the highest quality components in a brand-new system).
There are a bunch of threads in the Hardware Haven for this type of inquiry, as this really
isn't a technical problem, persay, but if you do have problems setting up the new rig (it's almost foolproof nowadays) you should come back and post specific problems.
I'm going to grab you some nice threads from the HH and you can use that as more of a guide.
Here's one on the "Best Choices for Motherboard, cpu, and ram": http://www.maximumpc.com/forum/viewtopic.php?t=1778'
Chumly is obviously an Intel supporter (as am I, really) but your son will probably be using this system more for gaming so it'd probably be better to stay the AMD/nVidia course.
As for RAM, I personally recommend Corsair plat XMS series; solid, stable, overclocks very
well. Others would recommend OCZ as well, but I've never had experience using it, so I won't.
As far as a video card goes, the BFG 6800 Gt is kicking ass in benchmarks all over ATi's counterpart the X800 PE, so you might want to think about that one.
Beyond that it's up to the rest of your budget, so chime in when you can.