THERE IS A SHORT VERSION AT THE BOTTOM OF THE POST

I'll be honest with you: from my experience, the "bulk cards" deals you find online are normally just dozens of dozens of duplicates, especially when purchased from individual sellers. The StarCity Games one, however, might actually be worth your time; I've bought a few similar boxes from a Vintage Stock near my home and been decently satisfied (but then again, I collect as many cards as possible regardless of what I get).

The Deckbuilder's Toolkit always has been and always will be a wonderful starting point. It comes with basic lands, which are very important to play with, as well as some random cards to start a player off. I might even recommend getting a second one of those, if you're planning on spending that much money on the game (maybe even one from an older set, for the sake of variety, though they might run for more cash-- M12 would probably be your best bet).

However, I might recommend asking your son if he's ever heard of "Commander" or "EDH". It doesn't matter what they are to you, but if he says that his friends DO play those formats, it is almost IMPERATIVE that you get one of the preconstructed EDH decks. The fun that comes within those boxes far exceeds, in my experience, anything else that Wizards of the Coast produces. There are currently nine or ten that would be easy to find, all probably under forty dollars (check your local Wal-Mart or Target, as they will always sell for ~$35 there). The decks are linked below.

But if he doesn't want to go the EDH route, the best thing to do would be to either A) purchase one of the intro decks or recent Event Decks (all of the recent ones are linked at the bottom). They are ready-to-play 60 card decks, and some even come with extra booster packs while others offer a 20-sided life counter die and efficient storage box. And if you feel like throwing even more money at his new favorite hobby (which I guarantee Magic will become), I'd recommend a Magic 2015 Core Set Fat Pack or a Khans of Tarkir Fat Pack. Both of these boxes come with even more basic lands (once again, very important), 9 booster packs, one very good storage box and two cardboard deck boxes, a 20-sided die designed specifically for keeping track of a player's life total (something not technically necessary but one of my favorite extras), and a helpful booklet explaining the rules of the game.

TL;DR: Go to Target or Wal-Mart or the local store that sells these cards. Get a DECKBUILDER'S TOOLKIT for sure, and then either A) an INTRO PACK or B) an EVENT DECK, and then maybe get a FAT PACK. But the important ones are the Toolkit and the deck.

EDIT: Fixed formatting.