For the account, your son will need an Oculus account that is linked to his Facebook account. I'm not sure if there's any gift card type stuff you can do, I don't think you can just load money onto it. You generally make purchases in the headset itself. You can also use the desktop app but that generally shows Rift compatible games. The Oculus account saves your CC info and you have to put in a 4 digit pin to complete a purchase every time you make one (it's kind of ideal for kids, since you can just keep control of that 4 digit pin and enter it yourself if you want to authorize a purchase).
If you have a PC that is powerful enough and a link cable or plan on going wireless with Virtual Desktop, then steam has plenty of games that are compatible. But that gets a bit more complicated in terms of which games are compatible, whether you use Link or VD, and whether your computer is strong enough.