The Dragons of Argent and Silver (Tales from the New Earth Book 6)

Seven years after modern civilization fell to magic and dragons, the remnants of humanity still struggle to survive. All humans have been Changed by the power, but a handful have gone beyond mere physical changes and are able to channel and use the magic directly. The greatest of these is Simon O'Toole.
Once a middle-aged man stuck in a dead-end I.T. job, now he is a young wizard with the fate of the world resting upon his frail shoulders. But something has gone wrong. Simon seems affected by a strange presence. His personality has been altered and now his friends fear that he has fallen under the influence of dark magic. He has cut himself off from the world and refuses to speak with anyone..
As the only wizard on the New Earth and the most powerful spell-caster alive, without Simon's protection the last of the human race may finally fall to the lords of Chaos.