Perhaps to best understand the title of The Reluctant Fundamentalist it is useful to understand the author's definition of just what fundamentalism is. Mohsin Hamid sees a fundamentalist as anyone who views the world through a single and narrow point of view. Furthermore, fundamentalism is not limited to religious ideology. When he first began working at Underwood Samson, Changez is very much a fundamentalist concerning the might of America's financial institutions.
When looking at it this way, Hamid sees the people who work on Wall Street as economic fundamentalists who see the world only through the narrow perspective of gains and losses on the stock exchange. Changez eventually comes to reject this sort of fundamentalism.
Where Changez becomes a reluctant fundamentalist is from the way that westerners treat him and view the Muslim world. We see this already from the suspicious manner that the unnamed American initially treats him when they meet at the bazaar at the beginning of the story. We see it more in the way that others view him when he is in America. When in the United States, people often suspect him to be a religious fundamentalist even though he is not one.
This constant suspicion, combined with his disillusionment with the American Dream, gradually but ultimately causes Changez to change his outlook. He comes to realize that as a Muslim and a foreigner he will never have full access to the promises of America. Even more, the anti-Islamic attitudes of post-9/11 America grate on him constantly.
By the end of the story, Changez has abandoned the fundamentalism of Wall Street and adopted the fundamentalism of his religion. He never set out to do this. Rather, he has been pushed in that direction, quite reluctantly, by a society that refuses to understand the nuances of his position.