Yup, he's going to need his own account and his own copy of the game.
The alternative is that you reset your game, losing all your progress and let him play from there. He won't be able to get any achievements you have already gotten (as you can't reset those), but it will still be enjoyable.
All that being said Duals of the Planewalkers is a fantastic way to learn. I used to be semi-competitive (far from pro, but I went to regular tournaments) and quickly was pigeonholed into one playstyle. DoP has taught me to effectivly use all the colors, and a lot of different playstyles.
The flipside is go pick up the DoP decks and play real magic with him! As long as you really understand the rules, and you're playing with the intention of teaching (not winning) it can really be an enjoyable experience for the both of you. The computer player will most always make the "right" decisions making it difficult for new players to see openings. Perhaps a combination of the two would be the best way to teach. I apologize for rambling a bit here, but getting kids to play games that require somethinking gets me going sometimes.