Well, I still don't know what he would be "building", but most brewers use a 5-20lb CO2 tank. A 5 lb one is like 1.5 ft tall and the 20 lb tank looks like a scuba tank.
Corny kegs are what the beer goes in. They have a reusable lid that allows for easy refilling and cleaning. You have to get quick disconnects (QDs) for the keg, one for gas and one for liquid (they are different).
There are two types of corny kegs: ball-lock (Pepsi) and pin-lock (Coke). Ball locks are generally more expensive because of the higher demand. Why higher demand? They are taller and skinnier than a pin-lock, so its easier to fit a bunch of them in a chest freezer (which is called a keezer when being used for kegs).
I personally use pin-locks b/c they were cheaper (cornykeg.com). But I sometimes wish I had ball-locks to make more efficient use out of my keezer.
Hope this helps, and I think the lively-hood of you and your son is OK. But then again, you do know him better than I do.....