This I can help with!
Is his Xbox connected to the internet already? Find that out. Do you have a wireless connection in your house? There's a wireless adapter, they run about $65-90. DH has his connected by cable, it's faster, but most people choose wireless.
Xbox live has 2 tiers: Silver and Gold. Silver is free, probably what he has already. Gold is the paid subscription, allows you to download content. You can get the gold subscription monthly charged to your credit card, this is the most expensive option. Last I checked it was abut $9 a month. DH was doing this for awhile until I realized how much it was costing us per year!

You're much better off purchasing the 12 month card, I wouldn't bother with the 3 month, they're usually half the price of the 12 month

.
Amazon has it for $45 right now, which isn't a bad deal, but often you'll find a better deal. Keep an eye on the deals websites-- I usually check Spoofee daily (I dont know if I can post the link, so google it) to check that day's deals. I've seen them for $25-30, which I'd buy that without hesitation as long as it's from a reputable source. (GameStop,
Walmart, Amazon, etc.) I'd also buy him a points card, so that he will be able to download game content like songs for Rock Band etc, or other games, or even clothing for his avatar. (LOL, I know.)
I don't know about facebook.

I'll say probably?
The keypad might be a nice little add-on. Typing messages with the controller is a pain!
Is he into the shooter games? (I don't know his age) Call of Duty Black Ops is coming out in November, it's *the* hot game. DH is drooling over it, he's a big Call of Duty fan. I don't think I'd recommend it for the 17 and under crowd, though. That's the mom in me coming out, DH agrees.
Another nice little stocking stuffer might be the Game Stop card. It's like their "rewards" type card, for $15 you get a year subscription to Game Informer (DH is thrilled when it comes in the mail) and also little perks and extra 10% on game trade ins.
Let me know if you need any more clarification or have any additional questions.