Listed below are the different products you may want to consider. All of them will be fun, and if you're not planning on surprising your son, I'd let him pick out his own depending on the kind of Magic he wants to play (he'll know the differences between the decks to some degree just by looking at the colors).

Intro Packs: These come with a ready-to-play 60-card deck and two extra packs of 15 cards each, as well as a box to keep the deck in. Any one of these would do fine!

• White M12 Deck
• Blue M12 Deck
• Black M12 Deck
• Red M12 Deck
• Green M12 Deck
• White M13 Deck
• Blue M13 Deck
• Black M13 Deck
• Red M13 Deck
• Green M13 Deck
• White M14 Deck
• Blue M14 Deck
• Black M14 Deck
• Red M14 Deck
• Green M14 Deck
• White M15 Deck
• Blue M15 Deck
• Black M15 Deck
• Red M15 Deck
• Green M15 Deck
• Azorius Advance Deck
• Izzet Ingenuity Deck
• Rakdos Raid Deck
• Golgari Growth Deck
• Selesnya Surge Deck
• Orzhov Oppression Deck
• Dimir Dementia Deck
• Gruul Goliaths Deck
• Boros Battalion Deck
• Simic Synthesis Deck
• Theros White Deck
• Theros Blue Deck
• Theros Black Deck
• Theros Red Deck
• Theros Green Deck
• Born of the Gods White Deck
• Born of the Gods Blue Deck
• Born of the Gods Black Deck
• Born of the Gods Red Deck
• Born of the Gods Green Deck
• Journey into Nyx White Deck
• Journey into Nyx Blue Deck
• Journey into Nyx Black Deck
• Journey into Nyx Red Deck
• Journey into Nyx Green Deck
• Khans of Tarkir Abzan Deck
• Khans of Tarkir Jeskai Deck
• Khans of Tarkir Sultai Deck
• Khans of Tarkir Mardu Deck
• Khans of Tarkir Temur Deck
• The Khans of Tarkir and M15 decks will be easily found in your local store. The others may need to be ordered online. I recommend getting the ones from the store, just because they'll be the least expensive.

Event Decks: These come with a ready-to-play 60-card deck with 15 extra cards to beat specific types of decks, as well as a box and a life-counter die. They're a bit pricey, but noticeably better than the intro decks.

• M12 Event Decks: I excluded these because they're almost $100.
• M13 Event Deck: Repeat Performance
• M13 Event Deck: Sweet Revenge
• M14 Event Deck: Rush of the Wild
• M15 did not have an Event Deck.
• Mirrodin Besieged Event Deck: Into the Breach
• Mirrodin Besieged Event Deck: Infect and Defile
• Innistrad Event Deck: Hold the Line This deck is a little expensive, but I bought one when it first came out and loved it to pieces. Highly recommended!
• Innistrad Event Deck: Death Fed
• Dark Ascension Event Deck: Gleeful Flames
• Dark Ascension Event Deck: Spiraling Doom
• Avacyn Restored Event Deck: Death's Encroach
• Avacyn Restored Event Deck: Humanity's Vengeance
• Return to Ravnica Event Deck: Wrack and Rage
• Return to Ravnica Event Deck: Creep and Conquer
• Gatecrash Event Deck: Rally and Rout
• Gatecrash Event Deck: Thrive and Thrash
• Dragon's Maze Event Deck: Strength of Selesnya
• Theros Event Deck
• Born of the Gods Event Deck
• Journey into Nyx Event Deck
• Khans of Tarkir Event Deck

Commander Precons: These are by far my favorite choice, as I believe Commander is the best thing about Magic. If your son DOES know what it is and says his friends play it, GET ONE OF THESE!

• Evasive Maneuvers
• Nature of the Beast
• Power Hungry
• Eternal Bargain
• Mind Seize
• Forged in Stone Note that this one should be bought at a local retailer for $35 instead of the upped price on Amazon.
• Peer Through Time
• Sworn to Darkness
• Built from Scratch Buy this one at a retailer for $35 as well.
• Guided by Nature